I have a 2001 Silverado 2500HD which has the hole above the mirror and I wanted to mount the faceplate/control head for my ALINCO DR-610 dual-band transceiver up there. After many trial and error attempts I discovered a project enclosure (box) which just fit the bill..... actually it just fit perfectly into that space. The box is a model JAL-4 which is 5 1/4"x 3 1/4" x 1 9/16" and comes from Hosfelt Electronics, Inc. in Steubenville, Ohio. The box was less than $3.00 and they make several sizes, this one being the largest of the series. I usually buy several of each size and keep them around for assorted projects.
I tried to find a web address for Hosfelt, without success, but did see two toll-free numbers and my guess is that a phone call to them would bring a copy of their latest catalog. Numbers listed are (888) 264-6464 and (800) 524-5414 and I have no connection with them, just a satisfied customer.
The box is molded plastic with an aluminum piece making up one of the larger sides. I was able to mount the front cover on one side of the box and run my cables through holes I made in the back of it. The box then fits snuggly in the Silverado's hole above the mirror. The fit is close enough so that it cannot rattle or even move around and rough riding have failed to dislodge it even a small amount.
Hope this is helpful.
Jim - K5LADjpk5lad@bigfoot.com