I would use empty boxes,preferably cabs to fill empty space.it will make it easier to dial in your preferred rh when you get to filling it up.
If you use beads or kitty litter Heartfelt Industries has a calculator for volume to beads ratio.I use KL and multiply the results by 2.5 times what is called for.It's not needed,but shortens the return to you desired rh level.

http://www.heartfeltindustries.com/calculator.asp