Because the case part holding the lenses of the optical mouse had a nice circular design, I went for cutting a fitting hole into the bottom of the SGI mouse. The standard hole saw have a drill sticking out in front of the saw, which isn't helpfull if the part already has a bigger-than-the-drill hole, ehich is tha case for a ball type mouse. So I needed to fix that by modifing the saw, by putting a threaded sleeve arount the drill (some tape to cover the tolerances):
Then carefull drill until the bottom part is completly cut through:
The rest is work with pliers and file.
After everything was fitting, I made chamfers on top and bottom oft both parts, applied tape unterneath and glued it in with 10-min-Epoxy, using a syringe.
So even, if the resin won't stick good enought to the other plastic parts everything staies in place, because ot the form. It is only needed to applied the resin on one side, it will flow into the bottom gap fast enough before curing.
And here is a picture of the new board inside the old case (some adjustments to the board like in the yellow marking). And in front of that the piece of the old board carring the three orig buttons fitting in the holding notches of the case: