Glue them (I use JB weld) to blocks of wood. Easier, faster and cheaper.
If working a rare piece use cedar and you can pick off wood by splitting off thin pieces. Clean the last bit of wood and JB Weld with a sander.
Way too much work and possible slips with a slab grabber in my opinion.
If I have a valuable end cut of jadeite and it doesn't fit perfectly with the grabber
(Note: You should always start with an end cut with a flat side)
I mark the end cut with a marker at the spots where the screws will touch. I then use a Foredom with a small diamond wheel to cut small grooves at the marker locations. This means I get a perfect or close to perfect solid fit with the grabber.
I have never had a end cut fall off in 30 years doing it this way.
Time to cut 4 grooves is typically about 5 minutes or less.
For less valuable rocks like agates you don't generally need to cut grooves as an insurance since 99% of the time the pieces don't slip if tightened correctly. Note again that I am talking about end cuts with a FLAT surface on one side. Trying to clamp a rock with a rough side against the clamp is problematic and more likely to slip.
I can't imagine using any other method to get the last few slabs off a rough. I also like the Comet design earlier posted. Looks simple, strong and effective. Maybe only slight downside is it takes more time to screw down and tighten.