re: I-beams

Those beams are just like the ones used for joists, great choice!
Nice thing is if you get your plywood web with parallel edges and your slots even in depth, you get nice and straight beams that can't warp, save a lot of weight, and actually can carry a significant load.

Hate trying to square ends on long boards, you need to make/have side table support and a good miter support. Forget having the wife or others help guide the board as you cut (at least in my case 🤠), never seems to come out right. 😱