Need a swim platform

I'm pretty sure the plywood was intended to make a pattern. Can use plywood, or I typically use trim wood and screw together sticks that are sitting perpendicular to the transom on one foot increments. Less cutting and fussing. None of this intended to be a final product....juts templating.

No, I meant plywood.
Hard to get anything over 8 feet long these days.
I make a rough pattern off of the whole beam of the transom in cardboard or construction (heavy) paper.
Say a 12' beam. will need three sheets of 1/2 or 3/4 exterior sheathing, smooth one side. One sheet cut in half for two 4'x4' sheets.
Laminate with mat, epoxy & lil bonding filler, the four sheets together with the 4' seems away from each other. use cheap screws to compress the sandwiches together. This is a messy job but you want that mat wet and the bonding epoxy to squeeze out of the sandwich nicely. Now you have a 1 or 1.5' thick laminated deck 12 x 4 feet long. Let cure a few days.
Now cut your transom curve on both long sides. Cut your taper to match the taper of your hull shape. remove all the screws. Thin some epoxy and start soaking it into the plywood edges. squirt some epoxy into the screw holes. This is important to seal these cut edges. Fair down the 4' seam screw holes & edges. Lay cloth all over one side of the platform and epoxy it down. Keep adding epoxy to the edges if it can still soak in.
2 days later flip the platform, fair the 4' seam, this will be the top. Fill in any last screw holes. Lay down woven roven and epoxy down. This is the start of of the deck surface you will stand on so lay it down pretty and neat. 2 days later sand down the edges. Here is your rough & sealed platform.
10 hours hands on, 3 sheets of plywood, box of cheap screws, half gallon of epoxy, pint of exciter, half tube of bonding filler, 8-5" sanding 80-100 grit pads, glass cloth, mat and roven, sharpie pen and heavy paper. $500.00 or less in materials. Not to bad for a 12 x >3 back porch.

Metal fab guy makes adjustable stainless brackets for another $400.00 (Max). Another $350 for finishing color paint and bronze mounting bolts.
You can ad non-skid to the top paint but if anybody is in dive suits or lounging on the back porch, I'd advise against any heavy application.

You can route out some wave slots close to the transom and seal them as needed.

I have made a few of these bomb proof back porches.
My diving back deck on my Trophy 2860 was 5' front to rear, full beam and held 3 full dressed offshore divers.
I have added to the factory platform on our Bertram another 2 1/2 feet (10 years ago). A lil weathered but still solid as ever.

For the DIYer, Coosa is only available to 8' lengths.
Custom cut orders are available for a whole one piece platform up to 12'.
No idea of the cost but if you want to avoid plywood on a budget, it's worth a call.

Say, That's a real nice AFT..
I like big classy AFTs.