I remixed the excellent gecko by WebmasterZero to include a spot for a 6mm x 3mm magnet.This prints great with or without supports on both of my printers. I may update the tolerances, but I have both magnets and suction cups that fit nicely with a dab of super glue. The cutout should be 7mm x 3mm.
I'm marking this thing as no longer in progress, because I've printed around 100 of them without major issue. Some filaments like to string at standard settings with a dense build plate, and I've had a handful of prints where the tail didn't want to stick, but it's printing well overall.
This model works wonderfully with 6x3 magnets, and also with suction cups! I have shared affiliate links to the specific items I used below if you wish to support me.
100 count 6x3 neodymium magnets: https://amzn.to/2kJTqhZ
100 count mini suction cups: https://amzn.to/2kJTxdp
Added void with tinkercad