Hmm it's not as simple as that? You would need a aluminum or steel mold, you would need plastic, you would need to melt it. You can't just make appear a random mold. And get good quality plastic bbasicly comme to the first problem find a corner. And make melt the plastic good luck?
And cn you tell me how you will make the mold? You need a material to do it, you can't just take an random material to make it. Dayan use aluminium molds if am right, witch are way less expensive then steel molds. And you can't really just make a mold from the corner, you will need to adapt the size logicly, and i doubt that dayan except to give a mold!
Im not trying to bring you down, im trying to give you a little bit of logic.
And yes you can make a mold out of rubber silicone and use resine to make the plastic (like tony fisher does), but it woulnt work well in a speedcube and it would cost more then getting a new cube
