My neighbor has one of those small Chinese excavators that has REALLY bad lighting... one small light on the boom.  So he wanted to add a light bar and the switch he has needed a mounting plate.  This plate was made in three variants because I was working out the attachment.  

all three models have an opening of 38.5mm X 21mm.  The mounting holes are 4mm through and 7mm recessed for the screw/bolt head. 

switchplate.stl - Has an overall thickness of 5mm and the inside wall (where the prongs catch) is 2mm.  It work but there is a lot of "slop" unless you want to put in a foam gasket (entirely possible and why I am including it

switchplate2.stl - overall thickness of 8.25mm and the inside wall on this one is 5.25mm.  This one holds the switch tighter but the prongs don't full engage.  If you massage the edges with a file you can adjust how tight or loose you may want it. 

Switchplate3.stl - 7.65mm thickness with an inside wall of 4.65mm.  This was the last version where I was just trying to get the switch to click in without a lot of fuss. 

The pictures shown are switchplate2.stl printed on prusament PETG in anthracite gray.  Printed on a CoreONE +.  Takes ~20 minutes on the CoreONE +