I added a hole for an 8x3mm disc magnet to Scott64's cool Tyrannosaurus Chip Clip (https://www.thingiverse.com/thing:1274014) so it can attach to a fridge. Even though it's a fairly rigid print it works really well as a chip clip (I use it mostly for bags of salad, actually)!
Just use some CA glue (or similar bonds-metal-to-plastic adhesive) to hold the magnet in place.
Tip: Rough up the side of the magnet that will be touching the glue a bit before gluing. It'll give the glue a stronger grip.
Shiny black PETG on the textured PEI spring steel sheet came out great... Looks sparkly!
I've also printed some of these in "high visibility" (so they don't get lost so easily or accidentally thrown out with the bag) 3D Solutech "See Through Yellow" PETG and that looks great as well.