My box is about 5' tall and 8' long, its built off my mason dump so a 4'x8' sheet of plywood fits right into the pockets. I have no vents. but the top only comes back one sheet of plywood (4'), so things "vent" all over if you over fill the truck - still its not really a problem though, and the nice thing is I can still get two yards of topsoil on the back, also without a full roof the tractor has more room to load wood/scoop out topsoil.
Yours will be a bit tougher due to the 12' bed. you'll have to stand three sheets up if you want 5' or taller, and reinforce them from the outside with 2x4's. You want the inside smooth, so nothing catches on it when you dump.
As for material; if you use pressure treated you will have to wait for it to cure before you can paint it - but I suppose it'll last longer. Last time (recently) I went pressure treated, it sucked looking at that yellowish wood till it dried. Whats the other?? CDX or some :censored:, you can paint that right away - probably good enough.
As for paint, I use benjamin moore iron clad black, its a satin so not too glossy. This stuff is also awesome on truck frames as its metal and wood enamel, cheap too. Use whatever primer the paint guy reccomends to go with.
The problem I had is, cuz mine is only 8' long, the new 250 was blasting that plywood up pretty good. To fix this I had a buddy that owns a sheet metal shop (HVAC) fab up a metal headboard. he even went around the 2x4's, then overlapped with the main sheet - looks cool too.
I really must need some more treework, lol. Hope this gives you some ideas though.