I designed these years ago to use with my laser cutter, and they've worked out great, so I figured I'd share them here.

The square and hold-downs have magnets to hold them firmly in place on my steel bed. They also work on honeycomb; however, they don't hold nearly as firmly.

Magnets can be found here: https://appliedmagnets.com/search.php?search_query=%22Neodymium+Magnets+3%2F8+in+x+3%2F8+in+Rare+Earth+Cylinder+N42%22

I built the standoffs I use, which I explain more about here: