I made a Monty Python and the Holy Grail themed backpack as a prop for a monty-python holy grail photo-shoot (hs senior pages). Simple and quick to make, they look really good and are surprisingly comfortable for being made of sticks and jumbled other things strapped on.
Step 1: Prepping the Frame
I used branches as the main structure of the backpack. This made it easy because they're simple to work with, have no need for any sort of perfection, and fit into the vibe of crappy backpacks for a squire or monty python and the holy grail. I used ~3 foot sections for the y-axis, and ~2-1.5 foot sections for the width. Measure the intended wearer of the pack to make custom (*gasp*) sizing, remembering that you should leave extra stickage at the top to get the over-sized look.
Step 2: Constructing the Frame
Use a file to create notches in the wood at the joints. similar to Lincoln logs, make the joint so the joint won't move when tied down. Use appropriate looking cord to secure the fame together.
Step 3: Straps
Pretty self explanatory, I used fabric scarps along with some more of the cotton cordage, and secured the straps to comfortable heights along the frame. Above you can see my two completed frames.
Step 4: Flags
Flags are important to complete the look, adding a sense of slight ridiculousness. I spray painted a flag design and hung it on a pole which I tied to the backpack, using wire to keep the flag flying nicely without wind. I suggest raising it substantially above the wearer to ensure a cool look.
Step 5: Decorate
This is the fun part. Grab all the shit you could think to look good to fit the era and tie it onto your backpack. Here are some ideas and suggestions:
Bedroll (this is #1, and i suggest brownish roll of cloth tied to the top or bottom of the frame)
Chest (an old timed wooden chest that you could somehow attach; I didn't have one of these)
Old leather bag ( I'm sure everyone has one of these lying around somewhere in the garage)
Basket/quiver (make sure any arrows are safely stored)
Sword/bow (again, thee can be/perceieved as weapons so make sure they're safe. in my case, i have a nerf sword and an unstrung actual bow)
Anything else that would look good and fit the theme, the actual movie backpacks have things like massive wooden boxes and even a small bench.
Step 6: Become Patsy (King Arthur's Squire)
Now that you've finished, wear it around like a true squire. DO NOT FORGET THE COCONUTS. I will have/have an instructables on costume/attire shortly. above are images where i doubled up both frames and props to make it look even better. Thanks!