You might try a hardware or home improvement store for some 1/2" "aluminum plywood cap". It comes in 4 foot or 8 foot lengths. You would need to trim the web to the size you want and of coarse drill everything, but taking on a build of that drill rig that should be a cake walk compared to the other tasks at hand.
I used this stuff for rails on my Knight Hauler, its easy to work with.