What you need is a drop bolt. Basically a usual slide bolt, mounted vertically so it goes down into a hole drilled in the floor. The slide bolt can't jiggle loose easily, and it'll hold the door firmly in position. You could even add a small section of metal pipe as a bushing in the hole, so it doesn't get hammered out of shape easily when you're bumping along a dirt track or whatever.

If you're feeling flush, two of those, top and bottom, should hold the door very firmly.