Morning Gentlemen.
My morning's thought is why not avoid the problems of making a mold that needs great care in surfacing/sealing/removing, by making a set of mitre-edged tiles in epoxy/quartz sand. Make each tile in a shallow mold, consisting of 1/2" thick battens laid on a polythene film surface, with the inner edges bevelled. Fill with sand/small aggregate, then pour over the epoxy. allowing it to form a self-levelling surface. You then construct the required block from the tiles, best surface outward, and back fill with the coarse aggregate mix, finishing with the final surface also self-levelling.
I've thought through a complete method, but offer this general principle for your consideration.
Regards
John
It's like doing jigsaw puzzles in the dark.
Enjoy today's problems, for tomorrow's may be worse.