I sold a couple of boxes to a mate at cost...a while back... perfect wooden boxes designed for transporting cats from cyprus... 
  
But i made a small..ish wooden box with a seperate sleeping area... and have kept them in that for 4 days with plenty of exercise and cleaning... 
  
Other than that,.. use a BIG cat carrier... 
  
Or... 
  
An old rat cage with a sleeping box...