Plastic containers OK?

I'm thinking that these would not be big enough, I've seen a lot of plastic containers and never have seen one as big as even a 2x3 c&c cage.

I don't know of any plastic containers that would be big enough either. I found coroplast with a few phone calls. I you can't find it near you there are other options like a shower curtin liner or a plastic tarp and then use the towels/mattress pads and fleece. You and your piggies will be happier that more room they have.

My previous cage was made out of 12" tall metal wire mesh expandable window screens and 2 underbed bins. We used tie raps to secure the corners. The end with the water bottles was made out of pegboard with tie raps through the board to make loops for the water bottle holder. The bins were set on top of utility carpet. The main problems with the underbed bins were the rounded corners and the height of the sides. The rounded corners did not meet and left a spot where debris would fall onto the carpet. Each time we cleaned we would lift up the bins and vaccum the carpet. Also when the piggies were small we had to use bricks to make steps where the bins met so the piggies could get over them. I think underbed bins would work as long as you were making a single story cage. You could overlap and tie rap the grids to fit around the bins.

Here are the underbed bins we used (without lids of course)
(broken link removed)

One of our guinea pigs chewed on the plastic for awhile. We gave her some well aged walnut branch to chew on and put black pepper on the plastic. She switched to chewing on the wood immediately. The wood was from our tree, aged for a while, then baked to kill any bugs before we gave it to her.