These holders are low-poly, easy to build "corners" that have a hidden compartment for a 0.75" or smaller diameter magnet. They're designed to hold 0.1" glass, plexiglass, acrylic, or anything you can fit in them. The models themselves are extremely easy to modify.
Simply print the corner "hole down" and print the associated cap. Slide your magnet in (available at pretty much any hobby shop) and glue the cap on.
I've demonstrated it here with an enclosure for my SD3. Even with cheap magnets, they're light enough to allow you to hold very large sheets of acrylic or plexi.