I too have been looking for ideas for CBT. I have found that the cable cuff at home depot can be used quite effectively to push the balls down. Watch the skin getting caught.
Good luck

I've seen a homemade ball crusher that was made of 2 sheets of plexi-glass with bolts and wing nuts to tighten it down. I didn't make it, so I can't give directions, but it was a very simple design with no built in limit to how tight it goes. The balls go between the sheets of plexiglass and then you tighten as you wish.

www.bdgsales.com/main.html

BDG Sales has a couple of models of an acrylic ball crusher. They're quite reasonably priced (buy the larger one, not the small, 4" x 5" one). This one seems to have a stopping point of around 3/8", or maybe it's 1/4"--It was hard to concentrate and measure it because the sub was yelling a bit.

I modified mine with strips of copper tape that run across the two inside surfaces. I hook up alligator clips to the copper tape and hook them up to my Erostek and I can e-stim the balls as I'm crushing them.

Actually, I have reached in and wiped capsaicin oil on the balls while they were being crushed and e-stimmed. Then I pulled on the ball crusher, so that the scrotum was being stretched as the balls were being crushed, e-stimmed, and hot-oiled. The I started tapping on the acrillc vice with a wooded stick. So, my poor(?) sub was getting 5 kinds of CBT at once. The ungrateful bastard didn't even thank me for using him so well.