I think you'll want a brace down the middle of the boat then several cross braces from side to side.
First I'd figure out how wide I want the floor to be and put a couple measured marks down each side of the hull where you think it should go. Then tape a string straight across between those marks, that will help you figure out how high your bracing should be (minus the height of the plywood).
Choice of materials for the bracing is yours, some use aluminum and some use 2x2s. Keep measuring and make sure it's close to level throughout as you go. You can use cardboard to make the patterns for the plywood floor sections when you're ready for that.
Then you get to decide whether to use carpet, vinyl, or some other coating over the plywood.
Decisions, decisions! :lol: