Using a plywood sheet for bed floor?

[floyd]

I have a 51 5 window that has a 3link rear and is bagged so using the original long bedwood boards isnt really possible. So my question is what thickness of a sheet do I need? Any idea what Id need to cut a sheet to (length/width) ,so when I installed it as the bed floor the bed would be square? My truck is a short bed.

I know Ill need the under bed wood side to side strips.

I tried to search it but didnt come up with much,so thanks in advance!