I could take some rough measurements for you. That would give you a place to start. -12s all seem to be a bit different from one to the next. I also noticed that mine are not symmetrical from side to side.
Ill start off with a a few pictures. These are my floorboards as I was disassembling my -12. Note the reinforcement for the front brakes is missing. The interior fabric is wrapped around the outside of the longerons.
The floor doesn't touch the longerons but is fully supported on the diagonals and cross tubes. The dimensions and diagrams I have are at the hangar. I may be there later this week or early next week and will grab them for you.
Even with dimensions you will still end up using poster board and foam board to build templates. And lots of duct tape. Or trace the ones you remove. FA Dodge sells a brake reinforcing bracket for the 12 that goes under the front floor board
Sorry for the delay, but life and weather have delayed my attempts to get the dimensions for the floorboards.
The aft floor is about 30" wide and 29" front to back. The front and back floors butt at the cross member under the truss.
The stiffeners are more like spacers that support the floor on the cross members and diagonals.
The ones on my floor are about 3/4" square.
as jrb said, start with posterboard or, cheap 1/4" strips of plywood and some hot glue and make a template from your airframe. YMMV.