While doing some research on this very probelm, I found the following article to be interesting.

I use an alternative method that works well, and doesn't have anything sticking out of the cowl that detracts from scale appearance, nor does it make it difficult to remove the cowl when necessary. I take a short (1/4 or 3/8) 2-56 socket head screw (Allen screw) and file down the threads with a cut off wheel on my dremel tool, just enough that it's a tight fit in the hole on the needle adjuster, and then tighten the setscrew down on it. With the engine mounted, I make a small guide from brass tubing just larger than my hex driver tool's shaft and glue it on a stand-off to the firewall so that it is in line with the needle adjuster and ends just before the inside edge of the cowl. All you have showing is a small hole, maybe 1/8 inch in the cowl where you stick your hex tool. The brass tube guides it right to the little screw in the adjuster, Voila!.