@grumblehound:
Thank you very much for the offer! But I guess I can find something similiar here in Germany as well, like this one. It's more expensive but with the shipping from England I guess I won't be any cheaper with your offer and that way I can even look around a little more and maybe get something that fits into the thick blade as well. But thanks to your suggestion I now know what I need to look for!

@BartElroy & FenixFire:
Well, my father actually has a (home) 3D printer that prints ABS. Since you listed this as one of the stabel materials I guess I could try it?
On a second thought, my boyfriend's father has a drimmel (is it called like this in English?) for metal so it might be even easier and more stable to just build the bladeholder from a metal pipe. :/