I'm getting fed up with the problems involving PzK, weird things happen, etc; and when I want to do something, it usually involves a lot of going back and forth and cursing.
So how are trenches done? I've made 3D foxholes; ala the trenches in the game; yet they don't "cut out" the map right.
Subtracting one mesh from another even with the full power of max is difficult enough to get right automatically (if often fails which is why it needs to be fixed up by hand), so there is no way that MM can do it and keep the map polys down. If you watch max do it will will crap itself and add about 100000 new faces and vertices to make it all line up.
So how can you do it?
Well build up your foxhole a bit and extend it so that it intersects a natual depression in the map. You will probably want to go into hires mode in MM so you have better control. Rob has done somthing similar for the infantry trenches.