I added a tach and had to mount it in the dash. A scrap of marine plywood, some epoxy fairing putty and a coat of epoxy paint and now TD has rpms to watch from the helm.
Cost? The tach cost a few bucks so it isn't part of the low buck project. The dash was from stuff in my shop.
I have asked Santa for an inexpensive automotive backup video camera ($30.00) that I plan to flush mount at the base of TD's pedestal facing the dash. My Garmin chart plotter has a video input I am not currently using. It would be nice to have the chart plotter's display include my A-4's instruments. It is worth a try. I will report back!