A what?Doesn't the case have a sliplock?
I found an old tamrac bag that should do the job if I take out all the dividers and use them to support the mount end.Otherwise the larger Thinktank bags have loops on either side for large lens cases and/or tripods, with a sack at the bottom to help prevent whatever you attach from slipping out downwards. Can't recall models off the top of my head though.
Bonus is they're also accessible from both the front and back and all internal compartments are accessible from two directions as well - within the bag or from outside.

