for the material on the drop in, i'd go with some plastic lattice, then for the tranny some outdoor carpet for control/ speed check. and for the rail, whatever u can find for cheap or free. I got some metal pipes for free from my dad's work, but otherwise, i would have gone with pvc.
When I built my box, i did a frame kinda like this:
on the top i put uhmw, but u could go with hdpe (both r expensive and if u wann make a box out of pvc use the same basic frame as the rail below, just wider...) and for the copin i used pvc rails
and the rail, like this:
(This is not me, but i build mine almost the same)
^maybe make it taller though