Tim - I know you're dedicated to steam gauges... And I know a cheap fix is desired. I'm going to add then to the unpopular suggestion that you consider not using this artificial horizon in true VFROTT operations. If you can't trust it 100% then you don't want to be flying with it.
There are electronic instruments being produced now which will fit in that standard 3 1/8" instrument hole and provide a huge improvement over your existing AI.
As well as some of the long-standing products like the Dynon D10-series. In fact I have a D10 that I'm considering liquidating. Most of these products will give you an AI to fill that hole in your panel, and will add airspeed, altimeter, VSI, T&B, skid/slip, heading (or GPS ground track) and other instruments into the package. By pulling your AI and installing more modern glass you are taking that one small piece of panel real-estate and using it to provide primary attitude information and backups for all your other six-pack instruments.
We've reached a point where investing time and money in vacuum-powered instruments is no longer a sound investment. Yeah, I know... it makes some of us feel a bit long in the tooth, but once you've flown with some of the new glass you'll wonder why you didn't switch to glass sooner.