There definitely would not be enough support for a one-piece enclosure if you have a square foot of opening beneath the drain area. Can't you dry-fit the piping and enclosure? You should be able to measure out from the surrounding walls to the center of the drain in the enclosure. There should be engineering/spec drawings giving dimensions and cut-outs available for your enclosure from the manufacturer/retailer.

I'm not sure about the supported along the edge part, and I'm having trouble finding any instructions on installing them, so I can't confirm that. It may well be true though since they are made from fiberglass.

Have you purchased the enclosure yet? I still think your best bet would be to dry-fit it, then you can transfer where the drain opening is to the flooring or a piece of plywood or cardboard if there's nothing left to mark on the floor. You'll also need to check out the instructions that come with the enclosure, as you'll probably need to attach the drain to the enclosure before you install it.