something like this ??
1/2" ply would be very okay. Screw and glue some 2x2" underneath so the ply
will sit flush on the bench seats, some Velcro Tape, some type of covering of
your choice, and you are good to go for less than $75.
go easy with the Velcro !! in large amounts, it is VERY aggressive !!!
just a piece of 1" x 4" in each corner will be very sufficient.