Sounds like you have a good thing there Hank.

I'd like to mention a few additonal suggestions for others who may be thinking of doing the same thing.

Consider using sand mix instead of mortar mix. It is usually a little cheaper for a bag, and should work just as well, if not better.

Unless you have an excess of epoxy resin, this could be made just as well with the cheaper polyeser resin. the polyester is much more resistant to UV, so you won't have to cover it with varnish.

If you mix enough pigment (or laser toner or graphite powder) in the resin to make it opaque you won't have to paint it later, and this will cover the use of scraps of fabric, too.

Before I filled the form with concrete or mortar, I'd position it in the boat, and scribe a line on the front and back to follow the curvature of the hull. I would make this line as dark as possible. After laying up the first layer of glass cloth and clear resin on the mold, I'd wait until the resin had set, but was still a bit soft, and then use a carpenters utility knife to trim the glass to meet this line. This way I have an edge that should perfectly match the curvature of the bottom of the boat. Even if I applied more layers of glass cloth and opaque resin later this should give me an edge I can follow in trimming those layers.

Attach the seat to the bottom of the boat with a fillet of epoxy ( no polyester here) and a strip of glass cloth, just like doing a seam on a stitch and glue boat.

If you need the seat to be stiffer you can fill the underside with epanding foam.

Now a request. Would you share the recipe for the mold release. please? how much soap? how much oil?

Paul G. Jacobson