I've got no skin in this game but...http://www.ebay.com/itm/400953265789
I found these puppies while looking for an excuse to use a discount coupon. Looking at the mount, the gauge can be taken out of the square plate and stuffed into a suitable round hole. Be very sure to use heavy (12-14 AWG) wire fused at the battery. The idea is to get as accurate a reading as possible. Long, skinny wires with poor frame grounds will make the true battery voltage seem distressingly low. The fusing is in case the hot wire wears through. Not Good.