I used the cutout in the right subpanel and made a pattern using poster board. I referenced off of known locations. I placed it on the left subpanel and drew the mirror image of the cutout.
I remembered that the vent for the pilot goes on the left subpanel. The normal vents have about a 1.250 vent opening. My friend Rick's RV8 that I had access to for four years, had a larger "eyeball vent" then the conventional vents in that the vent hole was about 1.625 which provides more air....and in the summertime that can be a big bonus!
I went to the Aircraft Spruce website to research vents. I found the standard ones and the big vent. Fortunately the Stein vents had drawings with dimensions that I could look at to assist making the clearance cutout adequate for mounting the 3" diameter of the vent. I then modified the cutout so that the vent will clear the subpanel and be mounted to the subpanel cover.
Then I drilled the corner holes and made the cutout. Next I filed and finished the edges. Left sub panel cutout complete. Next I will drill for the nutplates.