I've seen a few people use custom sized subwoofer boxes as the support for the cushion.
Starboard shouldn't be all that high. I ordered about a year ago, and I paid $65 shipped for the sheet of 1/2" starboard I used for the bow filler cushion support. I think you would be fine with 3/8" starboard if you wanted to save a little money. I ordered from Leon at Plastek Profiles 717-593-9200.