Unfortunately I've got no photos, so I'll try and explain best I can:
We had the same teak console, and wanted a tach, speedometer, water pressure and battery gauge within easy eyesight. In other words, not on the small vertical panels, that you'd have to bend over to see. I mentioned this project to a master marine carpenter who was maintaining my father's larger Chris Craft, along with some other families cruisers. He took some measurements, determined that the tachometer and speedometer were the same diameter, and the water pressure and battery gauge were same diameter too, but smaller.
About two weeks later, he stopped by and presented me with two teak "blocks" (for lack of a better description) that we mounted on the flat portion of the console, close as possible to the windshield, on both sides of the vertical center windshield brace. Each piece had an angled "face" that the instruments mounted into, flush, with radius edges that had been routed. The angle allowed for easy viewing. He had varnished them too. Cool.
If I recall, looking from left to right, they were water pressure, tach, speedo, battery. He made sure there was a certain symmetry to the display. The sides, back and front were 3/4-inch, and the face was 1/2 or 3/8-inch. Face was epoxied along with screws with bungs on top to the sides, back and front. So, essentially hollow underneath. There were additional wood cleats on both sides of the side plates, that allowed for two machine screws on each cleat to mount to the console. Washer and nylock nuts from underneath secured each one down. The wiring and speedo tube were routed up from underneath. Clean.
It got comments.
Since your console is fiberglass, you might consider using a composite material, like Starboard. Comes in a multitude of colors, probably Artic White or White-White would match the console. There are a number of suppliers out on the internet:
https://www.kingplastic.com/products/king-starboard/If I can get Jeff Rolfing onto this thread, he might be able to knock out a drawing of above.
Regards - Don McIntyre