Just wanted to follow-up that I used rubber well-nuts, like I posted above. I got these from Amazon:
https://www.amazon.com/gp/product/B0...re-bullets-btf
I used a #8-32 machine screw, which you can get at any hardware store. I used a 1 1/4" length, and it cleared fine, but 1" may work also.
The rubber well-nuts provide a seal, provide a rubber dampening for noise isolation, and will grip very well. Just drill the right size hole (~8mm or 5/16). I think it's a more elegant solution than sheet metal or self-tapping screws.
Tim