Did the research on retrofitting a gauge into the clock pod months ago already.
Firstly, smart doesn't just sell a pod. So if you mess it up, you have to buy the entire tach/clock kit. Thanks to Paul Bridle for confirming this.
Secondly, the pod can take a 60mm gauge though you
still need some kind of foam to fill up the spaces between. Tons of 60mm gauges in the market, Greddy, Defi, HKS, Autometer etc. The diameter is not the challenge but rather the depth of the gauge. The pod is about 2.4in deep and that's pushing it already. Look around in the market for a gauge that is 2in deep, cos you need to accommodate the plugs and wirings with the 0.4in left or so.
Technically you could let the gauge sit out of the pod a little due to the depth constrain. At that pt, you're faced yet with another challenge: aesthetics.
Installed a coolant + oil temp SPA Technique gauge myself. I went with a slightly different location than p1nball. Check
here.