Two things you can do:
 
1) Buy a piece of heavy fabric (like canvas) and staple it to the bottom
   of the box spring.
 
2) Get a mattress pad that has sides and put it over the bottom of the box
   spring (i.e., upside down).  The sides that wrap around the box spring
   are secured by the weight of the mattress on top of it.
 
Nancy and critters
[Posted in FML issue 3071]