Made a grommet..

[plum500]

Because I just have a little ol' automatic I made a grommet to feed the crank (#1), ground (#1), bulkhead power (#8), vac line, tac feed, and trunk solenoid line through the unused firewall hole... 2 pieces of 1/2" rubber and some industrial krazy glue I had left over from tire lettering... (there's another hole in it for the tac feed which I forgot when I took this pic). I shaped the plug part to go in the hole first, then made the backing piece... glued them together, and drilled the holes.

[70 sublime]

I had to make a rubber piece for a piece of furniture the other day and looked around and ended up using an old rubber hockey puck

Looking at your shape you made I think it could also have been done if you had a nice fresh square stone on a grinding wheel 
Just adding to the idea pool

[Dino]

Nice! I need to do something similar. I'm adding a fuse box inside the car and need to run power and ground wires to the battery. I should also add a hole for the tach as I'm already using the stock tach hole for something else. My car came with the standard clock.