Well you seem hell bent on putting "a" gauge there, so since your Roadster has a clock, coolant temp light and an oil pressure light (crude as they are - only tell you something's wrong, not what the exact value is), that leaves 2 options on your list that could provide benefit.
What can the gauges do for you?
Voltmeter = tells you if/when your battery is flat or when your charging circuit (stator, reg rec) calves
Oil temperature = temp of oil... unless you're racing, not much benefit?
I would also look at VDO gauges (they make the OEM ones) to get closer matching ones. However they can be a deep gauge and need a deeper pod to mount
I use AEM gauges, they have black face, white digits and orange needle, just like the OEM gauges. Nice and shallow fit
I'd vote for an AFR gauge, indispensable imo if you are doing any kind of intake/engine/exhaust mods