David,
A while back I contacted DC plastic about making a seat base for the '73-'74
250 Elsinore since they were offering one for the 125. They wrote back and
said all they needed was an exsiting seat base to do the deal.
You have an exsisting seat base for your Red Rocket so DC should be able
to mold one up for you. I have no idea about cost and production time but
I suspect they would absorb the cost in order to add another product to
their line.
If you don't find one, try them at
http://www.dcvmx.com It's worth a try.
dogger