After a slight conflagration incident, I was looking to replace my G2S Pro's Geeetech GT2560 control-board with an alternative. However doing so I didn't want to have to drill more holes in the printer, or be reduced to just gluing the Arduino/RAMPS/Stepper stack to the base.
Instead, here's an adaptor plate that uses the standoffs from a G2S (and probably any other printer using a GT2560) and gives you somewhere to mount an Arduino Mega and all the associated gubbins.
Attached are two versions - a standard one with no cutouts or anything, which will likely be strongest, or the skeletal version, the one in the pictures, which allows better airflow and is faster to print. And looks better, IMO.
(Scuse the mess, I'm still rewiring in the last photo.)
0.2mm layers, 0.4mm nozzle
After a brief clean of the 4 corner holes with a knife or file, the Arduino can be attached with short M3 screws and the assembly then just snaps into place on the original board's standoffs.
You may find that one of the screw holes on the Arduino can't be used, depending on the size of your screwhead; blame the developers, not me, apparently the holes were designed in before the board's sockets were added...