First join this cult
https://www.newschoolers.com/cultforum/453/Build-A-Jib-101
HDPE is what most mountains use
http://www.usplastic.com/catalog/item.aspx?itemid=23869&catid=705
Or UHMW
You can also save some loot and get pvc sheeting at home depot/lowes. Can also get lexan sheets there but that's not at all cheap.
If you're using sheeting you're going to need coping, that's where the pvc tubes come in handy. IF you're gonna drop some loot on good topsheet you should use steel for the coping. You can acor some angle iron down flush with the sides or leave a gap on the corners to fit a 1.5" steel tube. Or can throw a 2x4 on the outside set down and bolt it to the box. Worth it to get it put it in nice and clean and flush with the side though imo. Then you won't hang up on the edge and it'll look clean.
Also make sure you put it even or just below your top sheet. If you don't want to use tubes it's probably because you want it smooth for surface swaps and what not, that's pointless if you're going to snag on the coping.
Also don't run it without coping because that's dumb. IF you grip it you're going to be digging into the edge, cut it up, and hang up,and you're putting real loot into the top sheet wouldn't make sense not to have coping.
If you use HDPE and Steal coping make sure you spend some time and frame the thing out properly. You can make a rock solid box out of wood. If you're spending $$$ on material you don't want it super sketch.
Also leave a small gap on the sides and end for expansion with the changing weather. DOn't leave your box in the sun all summer. I guess if you're using it all the time whatever but if you aren't for a while cover it if you want it to last or else it can start to warp.
1/4" is generally the go to. Make sure to counter sink all your screws. Honestly a better bet to use bolts. Since you're rocking ply underlay make sure to throw a washer on each one. Also be careful, you don't want to pull them through, then you'll have a wasted hole that's always there that you can't use for a bolt. Make sure all the bolts are under the surface so you don't snag. If they aren't you're going to catch, and have a bad time possibly breaking you and your skis.
Def frame it in rock solid. Get your side rails(figure out what you need to do for coping or angle iron) then throw 2x4s across every foot all the way down. Make sure some are cut a tiny bit big and hammered in. That will add some rigidity. Basically just make it sturdy. IF you spend money on a good topsheet and steel coping you want it to last and not be shit.
Get a miter saw, some extra 2x4s and a little spare time.
