I make a lot of PCBs that are 70x70mm, and I've put M3 holes at the 4 corners, which are on a 60x60mm spacing.  This is an adapter to easily mount the board onto a Gridfinity plate.  The magnet holes are 6mm to allow a press-fit for 6x2mm magnets (the Gridfinity Standard).  

I chose to make it a split design rather than a single piece to allow access to the bottom of the board for soldering, probing, etc.  Also to reduce the amount of filament needed, and to allow narrower or wider uses.  

I used M3x8mm socket cap screws and M3x10mm standoffs.  You can adjust the length as appropriate, even omitting the standoffs. 

Background: 

The 70x70mm standard I'm using is because JLCPCB will automatically increase the board size to that if you have them assembling it. So I thought why not just standardize on that for most applications. 

Known issues: 

• There is a thin wall that doesn't quite print right.  I'll fix this in a future version, but it doesn't affect functionality.
• There is a very slight interference between the magnets and the M3 socket cap.  I could have moved the bolt hole, but that would have required me to change from the 60mm spacing, and that would have had some repercussions that would not be pleasant. As is,