Your mold looks simple to make, first off all this talk of never seeing polyester shrink is wrong, the reason why all this shrinking and twisting is happing is you never put cloth of any type on as your first layers, the twisting is caused by the fibers running down and across the mold will cause this very thing to happen, I allways use 1/12 oz matt as my first 5 layers of lay-up, one to two at a time, then after the part has had a good time to settel down use the cloth in small squares ond lay them and like bricks, this will not let the cloth warp the molds.
Been making molds for 20 years
The proper polyester resin to use is the ISO polyester resins, these are more tooling grade, and can be used in Exotic Materials parts lay-up. Also I have been using Orange Tooling Gel-Coats for molds and they make a better surface, they dry faster and less mold defects..
Mike