On my project plane, I'm considering just a airspeed/altitude/ ball & tube
slip (no needle) indicator/compass setup in order to avoid the venturi or
vacuum pump setup. Since I consider "electric" too expensive and wondered
whether a dome style compass might be the key??

Dear Dick,

The comment about "Electric" too expensive puzzles me. Electric DGs and
Artifical Horizons are pricey, yes, but do you have a starter on your bird?
Then an electric turn coordinator will keep you upright, as you turn in a
direction where your whiskey compass is actually a useful instrument. Every
Cessna built since about the time I was born (1959) had one, you can get a
yellow tag one for about the price of a pair of Air Jordan basketball shoes.
A brand new one is less than a clutch job on my Honda.

Cheaper than a grave marker.