Since I was not giving up on the base of my project, I went ahead and marked the hole for the beer shank.
I measured the width of the door and at the half-way point used the drip tray to scribe a circle where the shank would go. After centerpunching as close to center as I could estimate, I drilled a 1/4" hole through both sides with a jobber's bit.

The 1/4" hole in my fiberglass panel was the same diameter as the pilot drill on a hole saw.

The hole saw was chosen because it was a close match to a piece of sch 40 PVC I had left from a plumbing project. The idea was to make a PVC spacer for between the outer metal door and the inner fiberglass panel, since the panel would flex quite a bit when the shank was tightened. Also, Scott Mac had recommended that I move the insulation away from the shank so it could stay cool from the inside air.