Spring/Coil tool that will add pressure/force to support holding object in place
Use wooden shims.  You can use two of them (pointing opposite directions) to get a nice flat surface, and if they are not thick enough on their own, use more scrap wood to tighten the gap.

This solution is easy to tap into place and later remove with a small hammer, will give you as little or as much clamping as you need.  

If you need to move this item around a lot, just glue one shim in place (on the metal), so install and removal can be done with one hand.