I'm so excited that I found what I was looking for! My '86 Sunsport needs a new helm seat because the wooden core has just grown weak over the years. On Ebay of all places, I found a new-old-stock seat for a Mastercraft. It has evidently been in Mastercraft storage for some time because it's dated 2008 and was originally shipped from Attwood to Mastercraft in Vonore, TN. So for $230 I got a new rotomolded seat core with spring support and bolster. Also it includes ALL of the foam required, so all I need to do is take it to my local vinyl shop (I have the vinyl to match) and have it finished!