Doesn't seem to me that there'd be room for the whole mating/living/eating thing in a dog crate.  
My way to keep eggs pure is to isolate the roos I DON'T want to breed with my purebred hens.  So, if I had Barred Rock hens and several roos, I'd pull the Roos of other breed out of the flock.
  Isolate THEM for the 3 weeks for clearing extra sperm from all the hens for at least 3 weeks.  Then, put the purebred pair in another pen together.  (I have a moderately large pen for breeding and brooding inside the coop)  Once I have enough eggs to hatch, release the roo, or the pair if I were to use an incubator.  
Does that make sense?  This way, the spare roos get a bit stressed, but the hens are ALL happy enough, and the purebred pair (or trio/quad?) are only stressed for 8 to 10 days.  A friend says I should just leave them all the original way the whole time, but by putting the trio or quad in a seperate pen, I can be pretty sure the right hens get bred the extra bit... I mean, maybe the roo is too busy to get to the right hens otherwise! either way should really work!