Pop over to Home Depot and get one of the 2-inch "hub caps" shown below, or a piece of 2" PVC, cut a thin spacer ring, paint the inside flat black and glue THAT between the I/C and the gauge. Voila.OK so still trying to figure this out. Original youtuber answered back and said he just "glued" the tach to the back of the clock glass. That's not going to work for me. The clock glass is bevelled, and the little silver centre piece touches the gauge needle.
The tach, black bezel and all, fits snuggle into the clock opening. My plan is to paint the tach bezel a flat black so that it mimics the other gurages, and then take the silver centre piece off the clock glass and superglue it centre on the tac glass.