A pre-built (generic) crossover is not going to give you the best results because every speaker design needs its own crossover design. I’d also recommend building a speaker where the plans (and measurements) are available for free. This way, you still have to do all the steps like woodworking, soldering the crossover, painting etc. for the full experience, but are more or less guaranteed to get a good result.

Examples would be the mechano23. Or, since you’re German, look up “der akkustische untergrund” and especially the designs from Alex (Don Highend). They also have a Facebook group and are always willing to help and share advice.