they have canvas sides to put a bit of water on for evaporative cooling of water inside.
Tirdown straps
I would stretch 2 canvases separately and then attach them back-to-back, then maybe trim the edges for a finished look. If you need it thinner stretch onto both sides of stretcher bars and trim the second canvas to not overlap the front of the other side, then again, trim for a finished look - maybe hot glue a nice ribbon that matches your decor?
I don't know about your canteen, but the canteens issued to the military serving in artic conditions are steel insulated canteens. The looked more like the canteens carried by the cowboys in the westerns. The standard issue canteens would freeze.
An important new product is flexible fabric sides, called curtainsiders, for truck trailers. Another innovation is synthetic textiles that resemble canvas but are cheaper and easier to clean.
There are 1 quart canteens (32 oz) and 2 quart canteens (64 oz).
canteens. yummy
greasevile
Canteens were used during WW II pretty much the same way they are still used, to carry water.
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jackets
Information on how to create your own canvas can be found on Custom Ink, Canvas Design, Zazzle, Create Canvas, Murals Your Way, Easy Canvas Prints and Wet Canvas.
If you are using canvas board, you cannot transfer the canvas to stretcher bars. The canvas has been glued to the board.