Rain is exactly why I decided on my camper shell, not to mention air flow. I can haul planes in the dry and without air flow. As far as inexpensive shell goes, I think those words don't go together! I paid 1100$ for mine, of course it is color matched to my Red Pickup.

I made a wood rack for my shell. I can haul two planes on the bed of my truck, and two more planes on the upper wood rack (rails). Thus I can haul 4 60 size planes to the field at a time. (or 5 40 size depending upon actual size)...