I made some negative form 20 years ago with my self built cnc and self written software under DOS.
I made some very light wings for a hand launch glider which were shaped eliptical, so the wing tips were bowed and cut back eliptical. It flew rather good. But at the first try i had a severy problem:
I built the forms of styrodur, covered wih epoxy and milled again, sanded, polished, applied wax solution and that was a big mistake. There were small cracks in the epoxy and the wax solution ate up the styrodur beneath. I still managed to use the forms somehow and made my first wings.
The next form was glas fiber reinforced and i applied thickened epoxy to give some meat for the following milling process. But also here i had to epoxy a second time to get rid of bubbles and mill once more, a process taking up to 14 hours without interruption.
The result was amazing: I got a complete hand launch glider with under 200g, the wings weight was below 100g, at a span of 1.5m. The wings could be twisted against each other to provoke rolling, or together, so the tail was completely passive, without any flaps, very light, so the plane was very responsive.
Later i made a flat negative wing out of some nylon or PP on a mill. That was very sucessful. It was a middle part fitting to the hand launch glider wing and i built a 2.25m span glider with twistable outer wings.
This material was nice to mill and to sand and polish and it was separating perfect.
I also used Poly vinyl alcohol as an additional separator. It is a solution, but alcoholic, not eating up styrol. It builds up a closed film and is water soluable afterwards. All forms treated with that were unforming without great effort and gave a perfect finish.
I mostly used 90 minutes epoxy at that time. I used to press the forms together with M10 threaded bars and around 10 bar clamps and large wooden support bars. I estimate the force to 1500kg.
I always let cure for 24 h. then i started a heat treatment by hanging the complete construction above an oil fired oven to reach a temperature of around 70°C for 5-10 hours. Cooling down inside the form is also a good idea. Let it rest for some hours before you start to separate.
Especially for continuously bended wings this directly build of negative forms has potential against the traditional way of building a positive form first. It is easy to process the parting surface in the mill as an integral part and its precise.
Please try it. But i recommend not to use foam and glasfiber. Its a health issue to mill and sand glasfiber. I let a vacuum cleaner run the whole time when milling and i sanded under water only. Invest in a thick nylon or PP sheet and reinforce the back with at least 20mm plywood.