I had a bunch of 6mm magnets laying around, so I modified JayFi's original in Tinkercad to have larger holes for the magnets.
25%, not all that critical
It's pretty flexible when printed in PETG, have not tried PLA yet.
I made the holes 6.5mm in Tinkercad and they ended up being a snug fit for my 6mm magnets, they're held in by friction for now.
Otherwise I imagine some CA glue or even hot glue would work just fine to keep them secured.