My Willys is a plastic car and has a smooth firewall. I too did not want any holes in the fire wall. I mounted (glued) a piece of 3/8 Birch ply wood to the inside of the firewall. It has a number of 1/4-20 blind fasteners so I could add a second 3/8 Birch panel. Purpose being that if i really didn't like what the second panel did, I can sort of easily change it. Everything mounts to the plywood.

I even have the ignition box and coil mounted to it. I have an MSD pass thru for the coli wire that you can hardly see under the hood. The only other hole inthe fire wall is for the throttle linkage and it has the Lokar cable running thru it.

It is a lot of extra work to do the clean firewall but I feel it is worth it.

My car is not very big under the dash so there is a huge amount of wiring wrapped up under the dash and in the kick panels.