Use GtkRadiant ;D
http://www.qeradiant.com/

You can make tubes and stuff as a brush, and not as a model. That way, you can see how it looks as soon as you make it.
But I guess you do need models for the splitters.
But Milkshape can export as a .MAP file right?
And of course, I probably don't even know what's going on, right?

6 nCr 1 = 1
6 nCr 2 = 15
6 nCr 3 = 20
6 nCr 4 = 15
6 nCr 5 = 6
6 nCr 6 = 1
-------------+
58

Explain in layman's terms, please.