i had to cut mine...
you can see the gap inbetween the stringers and transom (i have an i/o so i need a 2 1/4" transom... you wont have this problem)

my first piece went in solid

but my 2nd piece wouldnt fit with the lip on the top of the boat so i cut a small section off the side to get it to fit

then it got all glassed in

since yours is a jet and you're not doing it for strength just something to attach stuff to i dont see any reason why u couldnt trim it to get it to fit, its going to be VERY snug. 1/2 or 3/4 ply should be plenty. just be aware when you pick out screws to bolt stuff in.
it looks much better than the old transom
