I made a template by taping a splatter target to the inside of the light then trimming to fit the edge of the housing after making cut out for harness and antenna connectors.
The clearance for the connector will need to be opened up 1/8" on all sides for connector relief plus a bit more for the locking tabb.
Poke holes through template for mounting screws.
I measured a light bar on an Ultra in the show room. Told sales manager what I was doing and used a cloth tape used for sewing so as not to damage anything.
Tape off the mounting area with painters tape then fasten template to TP and measure 3 times to ensure you have it level and even side to side before drilling then measure it again.
I used 1/8" bit for pilot holes and a Roto-Zip for the harness connector opening.
Used a step bit for antenna connections.