I built a bed cover for the 64 I had way back in the 80's. I pulled into a WalMart as a business looking guy was walking past. He said he really liked it and asked about it so I told him how I constructed it.
A couple years latter I noticed the fiberglass covers on truck beds... Hmmm...
Anyway, I'd probably build one for these trucks the same way.
Explaining how might be the tough part.
It would start with a grid work of 3/4 plywood strips about an inch and a half wide, spaced about 12" on center. The strips going front to back would be solid, the strips from side to side would be cut and fit in place, offset to make fastening them in place easier. The outside dimensions would be about 3/8" shorter than the width and length (inside the bed) measurements. Once you have this grid built, apply a 1/4" sheet of water resistant or exterior grade lauan plywood (If they still make such an animal.) to the top and bottom side of the grid and then sand the edges so they are slightly rounded over.
I sanded and then primed the wood with rattle can primer and painted it several coats with rattle can paint that matched my truck color.
For this lid to work with this style of bed, you would need to put a cleat (1X2 ?) inside the bed at 1 and 3/4" below the surface where the top of the bedsides angle outward. (And only on the bedsides.) This would put the top of the new bed cover even with the top of the tailgate.
On the one I built, I used regular door hinges to attach the front of the lid to the front bed panel allowing the lid to lift up from the back.
Now they have all those gas filled arms that you could install to hold the lid up while loading.
I'm sure with your creativity, you could devise some kind of locking device.
So building a lid like this is cheap, substantial enough that it supports itself allowing the tailgate to remain operable as is, and allows you to paint some way cool design on the top side and even the underneath~ say if you are at a car show or something...
Keep in mind, other materials could be used obviously. I just like cheap and something I can build.
