After perusing all options on the threads here, I decided to take a stab at it myself. My goals were as follows:
1) Minimal modifications to the car.
2) Easily reversible.
3) Should not block any other gauges.
4) Should not look tacked on.
5) Easy and quick to read.
6) Should retain the AC air flow.
The left vent gauge come off very easily. Use a pry tool carefully and pull out with your fingers.
Vent:
There are four pieces that constitute the vent. They are held together by locking tabs which separate very easily.
Take the part with the vent flaps (third one from the left in the picture above). The actual flaps are simply slotted into holes in the side and act as a pivot. Pull them out. There are two parts.
For the next step I used automotive weather seal (thick rubber) and I cut up a grippy rubber mat that I had. The type that you put on the car fender when you work on the engine. Stick this on the sides as shown.
Insert the gauge. It will be quite snug so you may have to push hard. Mine was quite firmly held with no play.
You can vary the length of the weatherstrip to adjust for air flow.
Test fit by reassembling the parts. At the same time, drill a small hole where I have, to run the wires through. The location of the hole sits outside the vent tube in the car so the wires will not be pulled in. Seal the hole around the wires. I used some Dynamat.
You can also rotate the gauge in the vent like the stock one as long as you left some play in the wires.