Today I drilled a couple of 3.5" holes in my jeep
First I pulled down and cut away some of the material covering the area to be drilled. The following image shows the bare firewall with an oval hole already in it and 2 other things sticking out of it, all three of these will be included in the hole to be drilled an will no longer exist.
This was a pain. If you do this, make sure you have a long enough drill in the center of your hole saw so that it reaches through the oval hole and hits the wall behind, this will keep your drill steady as you go through. (I figured this out the hard way
Also, notice how I chopped off part of that dented-outward part of the firewall, I recommend avoiding that and drilling higher than I did. I wanted to avoid it but it didn't turn out that way. In the next photo you can see that because I didn't drill higher than I did, the hole drilled in the back wall (being curved) isn't quite circular.
I ordered some rubber grommets from Minor Rubber (I actually had to order 34 of them because of a "minimum order". But I couldn't find these the size I wanted anywhere so I went with it.) The following image shows the rubber grommet placed around the edge of the back hole. I added some silicone sealant to the inside of the grommet groove.
Then I placed another grommet on the front hole.
This image actually does the result justice. Because the edge of the hole is 1-3 sheets of metal in different places, the grommet doesn't fit nicely.
I'll have to do some forcing to get the pipe through...but at least it'll be really snug.
This part of the project is less than perfect, I didn't expect the walls I was drilling through to be thicker in some places than others.