Mount for Mini dc voltmeter. I bought a few from China on ebay last year. I believe it was this one (I have updated this link because the old link was bad--this replacement should work):
tinyurl.com/47pmk282 _trksid=p2047675.c100005.m1851

This is a nice tight fit when I printed it using PLA

<> I added a cover piece with mounting tabs

The dimensions of the hole are 10mm x 23mm approximately