Two ways I've seen large-format drawings stored: hanging vertically.

If one were woodworking-inclined, it wouldn't be too difficult to make a flat file from some kind of engineered wood (plywood, MDF, etc.) and some drawer slides. But the easiest by far would be the vertical option. I'd think if your papers could stand some pressure on the edges, that you could hang lots of stuff vertically with a combination of side clip report covers and some string or wire. You'd probably want to superglue a strip of something stiff across the top of the clips so that they'll act like one giant clip. Add plastic bags from the dry cleaner if you needed dust protection.