homebuilt truck tonneau cover - plywood vs. rigid insulation

[hpcbmw]

I've got a 2000 F250 shortbed pickup. I need to drive from NorCal to Oregon next week to pick up a bunch of nice furniture. I have borrowed a 10'x6' enclosed cargo trailer, but don't think everything will fit. Being from California, I'd normally just throw the rest in the bed of the truck and call it good. However, the furniture is in Oregon, so of course it's gonna rain.

I'm thinking of building a simple truck topper, not a camper shell more like a tonneau cover. My bed rails (top of the bed) are about 80" long by 70" wide. If I cut plywood into 2 40x70 pieces, lay them on top of the bed rails, then run a 6' 2x4 over each one and bolt that front and back to the bed rails, that would create a relatively secure and dry space underneath. I could run some tape over the seam to make it a little more water proof, but I expect water would seep in .

My second thought - use rigid insulation like this
http://www.homedepot.com/p/R-Tech-1-1-2-in-x-4-ft-x-8-ft-Foam-Insulation-310880/202532855?N=5yc1vZbaxx
and do the same thing. This should create a tighter seal and taping the seam should be more secure, hopefully keeping more water out.

Anyone try anything like this? Any ideas why it would or wouldn't work? I'm trying buy a cheapo camper shell (under $100) just for the trip, but haven't found many that would work on my truck.

Thanks for any ideas!

 

[ntsqd]

If it is a one-time use then I'd go with the lowest cost option, what ever that might be. Are those FRP bathroom panels less expensive than plywood?

 

[hpcbmw]

I have some of the insulation panels, and I think they would be more water tight, so I'd probably use that. However, I found a couple camper shells on craigslist for $75. I'll probably buy one for the trip, then re-sell as soon as I get home. Plus, I found out there is even more furniture then I thought, so I'll need the extra room of a shell vs. a tonneau cover.