Tom,

I suggest that you saw off a few 1/2" cross sectional pieces to shop around with. Mail them to a few yacht suplus places and ask them if they have a similar extrusion. If you can find the same extrusion, then your base will fit into it.
Minnies in CA
A list of yacht surplus places

I thoght that the Freestyle looked a lot like a Supercat. Even had the same rudder stocks, didn't it? Maybe Aquarius knows something about where to get parts for it. Aquarius SuperCat

Doesn't you boat look just like This SuperCat 15?

GARY