Here is a few pictures of exactly how I went about attaching the gauges to the dash.
I bought these from Canadian tire for something like $1.50 each.
From there I took a dremel cut off the 2 pieces lengthwise as shown here.
I used a dab of superglue to hold the nut aligned on the middle hole of the metal piece. Once it was tacked I JB welded it so that I would not have to hold it from spinning when clamping it down on the dash. I then bent both ends of the metal upwards to hook under the dash. I used a 3/16" screw that was 3" long (I wouldn't go any shorter).
I then drilled a hole in the top of the gauge cover one size bigger than 3/16".
I then sanded down the top of the head of the bolt to try and get it more flush so that the hat would sit evenly on top of the gauge. In order to sink the bolt head into the cap I ended up going with a 5/8" (or so) drill bit and went down about 1/16 of an inch into the hole I drilled so that it created a dish that the head of the bolt would sit in and bring it more flush with the gauge mount.
I was able to get it pretty close but I also took a flapper wheel to the underside of each hat to get it to fit perfectly, then I just put a tap of silicone under it to hold it. And the finished thing. Minus the crappy iphone pic.
