Hi beat, I think it's hugely more difficult to design a (good) crossover than it is to build someone else's design. The former means a long road of education and experience. (I assume, I am only at the start of it myself...) It's a lot of fun to learn new stuff, but if you're more looking to build jiust one or two pairs of kick-ass speakers and then be done with it I'd be inclined to look at designs or kits from the more highly-regarded designers who publish/sell that kind of stuff -- Matt Dillon, Dennis Murphy, Rick Craig, Danny Richie come to mind.
HTH