> The problem is you can't just dump a bunch of resin in a mold and
> expect repeatable results. You need to get a decent low expansion epoxy, ...
I have to add a bit here.
Yes you could, if the mould itself is stable and precise. Epoxi (or epoxy) is not expanding nor shrinking when its hardening, and it doesn't expand when it gets heated - but it softens. But to make sure you get the strongest piece out of your mould you must have a heat resistant mould that can be placed in an owen, epoxi that is heat treated (it's called "to push it") during the curing face gets a lot stronger and more heat resistant. I've been making sheet metal pressing tools for the car industri in plain epoxi running for years without any fatigue.
Epoxi is a fantastic material, but I think it's not the best price-per-performance-solution for a hobby machine. But if you decide to go for it, be careful with the liquids because they are extremely allergic developing. Good luck!
Cheers,
Sven