I've seen the other locations that people have done but I think I'd quite like a gauge or two in the drivers side vent. Fit a blanking plate ala barnacles widget and put one or two gauges in there.

Anyone done this already?

And how do the barnacle widget plates fit in the vent hole?

barnacle widget plates are a very well cut bit of plastic that have no fixings just a very snug fit .

ah, are they flat or shaped at the sides then?

No they're flat, rounded at the edges to match the shape of the hole.

Any guage you put in there will likely be 2" / 50mm diameter and to make it fit you would need to enlarge the hole slightly, since the gauge would need to be angled up slightly, any change might be irreversible, so think carefully.

I've put my AFR meter in a pod mounted upside down and stuck on the bottom of the dash (with some screws to make sure it doesn't fall off). After the initial novelty of looking at it constantly wore off, I didn't need the AFR meter to be visible all the time, and didn't want to drill the dash nor have a 3 pod A pillar thing (already got 2).