I built a case that held a Sinar P 4x5 upside down by the rail out of thin plywood, glass fiber cloth, and epoxy. I made a rubber gasket to go between the lid and the main body. It was ugly as sin, but it was cheap, sturdy, light and highly water resistant. I used to bungie it to an aluminum external back pack frame for day hikes.
These days I use a Toyo AX in a Photobackpacker backpack for that sort of thing.
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