I rebuilt my l-lounge on my 238 which is similar but I used the old rotted one to start, traced all holes after removing hardware. then I assembled the seat back to the plywood only and placed on the hinges to mark where mounting holes go. I guessed larger where the rotted section was and trimmed to fit exact
there is no exact measurments you have to do a dry run to make sure it fits properly.
mine was cut from one single sheet of 3/4 ply and there is also a 1/2 inch ply build up area where the hinges mount
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"Changes In Latitudes, Changes In Attitudes "

1996 238 Vista dlx "Casper's Toy Too"
2006 5.0GXI w/closed cooling, DP-sm 1.95 drive
Previous boat:
1998 Sundowner 205