I asked that question a couple weeks ago, this is what someone suggested to me...
"I used the Autometer gauge (Summit part #ATM-2173 $29.88) along with the necessary adapter (Summit part #ATM-3280 $7.95). I use a 90° adapter that I got at the hardware store.
Just checked, the 2173 (black face) isn't in stock, but the 2179 is. Same thing, different color (silver). 2176 is the (also out of stock) white faced version."