First off it should be noted that medieval people did not really have back packs as we know them. Most of the people being tied to the land and a lord, they did not travel on the weekends to see the sights.
That said; there were people that went on pilgrimage and walked hundreds of miles to visit holy sites and cathedrals, but they carried very little, relying on charity to get by. The average pilgrim likely only carried a bowl, spoon, water costral and a few odds and sods in his scrip, or bag. Wealthier people may have had saddle bags and pack horses and of course there were wagons, but medieval people did not carry the kind of gear that modern hikers and campers carry.
The main reason for this is of course that camping is a great way to get knocked on the head, robbed by an outlaw or killed. Also it was illegal through much of the middle ages to be “at large without patronage”. A fancy way of saying that if you were not staying at the inn, the church, or a local barn, you were likely going to be tossed in a dungeon until you explained yourself and paid some lord compensation for camping on his land. Of course this is one of the reasons why there were inns every day’s walk across Europe in the later middle ages. Also being superstitious; to most medieval people, werewolves and demons were real threats, and at one time Europe still had some predators capable of hunting humans. Most however have been hunted to extinction since then.
Camping and hiking then is mostly a modern pass time, that became popular at the end of the 19th century. Still however, there were lots of people traveling the roads in the middle ages both legally as merchants and free craftsmen and illegally as outlaws. They had a number of methods to carry their gear, tools, or products, should they be poor enough not to own a pack animal or wagon.
The most common backpack device is the pack basket. This was primarily used by workmen, market gardeners who may have had some surplus to sell, and the common man that needed to move a lot of volume without a draft animal. Pack baskets were used well into the 20th century on canoe expeditions and can be made very robust.
The Pilgrims bag is shown in many illustrations. It can hold a fair amount of stuff; a bit of food, some eating utensils, and a few small items but they fill up really fast. You may be able to stuff a blanket in a big one but they are more for the person on the go who has accommodations for the night.
The scrip or sausage bag was a good way to carry a few more items than a pilgrim bag. They could be made as large as needed and had a single strap similar to a shoulder bag. Leather versions could be waterproofed to an extent to protect dried food goods if needed.
The Martebo sack is another simple way of carrying a good amount of gear in reasonable comfort but it is not to be over loaded or off balanced. One side needs to weigh about the same as the other like saddle bags for a horse. At best you could but some extra under clothing and some travel food in one. You are not going to carry your merchant goods in it. Still it is a great little day bag.
The most common way of taking a lot of stuff in the medieval era was to bundle it up in a tarp or blanket. A traveling merchant that could not quite afford that horse yet may have to do this. Wool was also processed into bales and was cumbersome. For the man who had to carry bales of product or gear, a pack frame would go a long way to making his life less hellish.
While it is perfectly feasible to carry a bundle short distances by roping it you your body, this is not realistic over long distances. Medieval people did what needed to be done of course but we in the modern world are used to a better life and not dieing from preventable illness and injuries, so there are better options. Bundled gear tied to a pack frame shifts less and does not give you rope burn.
So if you do not want to make a wheel barrow or a hand cart but still need to take some of those modern items of comfort with you on your medieval hike or across the field at the local SCA event, a pack frame may be for you. They have been around since the neolithic era and even Otzi the Iceman had one. There are a few different kinds but the easiest one for a medieval traveler or merchant to construct is basically three sticks tied together.
The pack frame I built is not based on anything or even a pattern. Other than items I had in my scrap bin, it is just 3 sticks tied together. I spent some time carving an ash wood lumbar support with an axe and knife as I wanted that area to be reasonably comfortable, but the uprights are just cherry wood sticks. The frame is lashed together with goat rawhide I had from another project. A medieval maker would likely use rope or home made cordage from plant fiber.
The top attachment for the straps is just a circle of leather cut from the outside of a disk, It is fitted over the top and pulled through the uprights above itself. The straps are sewn to this. A leather thong is attached near the back rest at the bottom. A hole is cut into one end of the thong and the free end passed through the frame and then the hole to form a slip knot of sorts.
The thong just passes through a piece of leather sewn on the straps. A knot can be tied in the thong to adjust the straps. The thong can also be used for a rudimentary hip belt if tied around the waist. The straps themselves are folded canvas from an old canvas tarp that got destroyed and recycled. While medieval man was willing to deal with rope burns and pinched nerves, I am not, and so nice wide straps take some of the pressure away from the load and distribute it over a larger area.
With the gear tied on, in this case a canvas tarp to keep the rain off, a modern self inflating sleeping pad in a canvas bag and a wool blanket, the frame weighs 17 pounds. Not to heavy at all but way more than anything a medieval pilgrim would carry.
This type of construct would likely be limited to hunters, poachers and foresters who needed to go out into the forest and possibly carry back a quartered red deer for the kitchen, or to the lower income merchant who traveled without a horse or hand cart. For use in the SCA, be that on a hike or at an event, crossing a large field with gear, the lightness and versatility of a home made pack frame is hard to beat. You will not see the knights in shining armour using them, they have squires for that, but for the Forester it is just the rig to carry your gear in some comfort and plausible period style.