Drill two round holes in the chassis that will fit those cast protrusions that hold the M6 threads. Buy M6 fiber fender washers, remove both nuts from both studs, then use the non-conductive washer to clamp the cap to your chassis plate.
It would be worth measuring those protrusions individually, as the manufacturer may allow for some variation in their exact position.
Your other option is to get a clamp and cut a large diameter hole in the chassis for the body of the cap to slide through.
-PB
