Have you built your own custom sink out of plywood and epoxy?

Making a mould sounds like a good idea if you can get a nice interior result, which is what a sink is all about.

It also gives you the opportunity to use gellcoat or similar product as the first layer, which has always been a better way to go for long term quality finish.

BB41 comment on saving all that interior cavity sanding really rings bells with me. Its a pig of a job to get done properly. Doing a mirror finish on a male shape wouild be much easier.

I remember a few people on this forum have made successful moulds from cheap mdf and plaster and builders bog. There probably isnt any real reason to need plywood. As mentioned here 200 gsm, some matt for the hi-stress areas and polyester resin over the gelcoat would do a great, lightweight job.