I have a big trunk where the back seat was, and it hangs over the bags so top opening was not a option.
BUT with a couple old ammo cans, some fiberglass, foam for a mold, and some building time. I now have some bags that are big, tough, and WATERTIGHT!




Rolled on some truck bedliner for a finish and I can pack all kinds of stuff now.

I started with a standard 50cal can, cut out the bottom, stood it on end, built a mold from blue building foam, covered the foam with clear plastic wrap, 2 layers of glass on the outside, pulled out the foam, a layer on the inside, then 2 more on the outside. Didn't feel like sanding and filling the glass smooth, so just rolled on 2 coats of pickup badliner.
They don't look pretty and shiny, but they are tough and watertight.
If I did it again I would use ''Fat 50'' cans. They are 1 3/4'' wider and would give me about 30% more space inside. The wind drag is not really a issue as they are in the ''dirty air'' from my feet & legs.
The pins thru the latches allow me to lock them up with small padlocks when nessessary.

I like!  Shows imagination .