Okay friends, deassembled the large metal-profiles because they appeared to massive. And yes, the mold had a better look afterwards. But guess what happened when I filled it with plastic flakes and screwed it together ?  The mold bended, but with continuing of the baking process, it got back in the original shape. So the first attemp to make it more stable was not that wrong. Anyway, I will go in the direction of a lighter Verison, to reduce the thermal mass and the baking time. So stay tuned