Can anyone recommend suitable equipment/method for a temporary OAT gauge, for flight testing a single seat experimental category aircraft that doesn't have one. I would rather not install a permanent OAT if I can avoid it, it's an aerobatic type with a minimalist panel.

Personally, in an aeroplane of that class, I'd not bother.

I'd for preference use ground OAT and assume standard degradation with altitude, failing that or use the values from the latest Met Forecast.

For the 1-2 degrees error that *might* give you at altitude, the effects on the flight test data of an aeroplane in that class, the errors in any performance reduction will be trivial. (Realistically, the only really critical performance numbers for your aircraft will be take-off and landing, where a ground recorded OAT will be fine.)

Just avoid doing any performance climbs through, or above, an inversion - but there are plenty of reasons not to do that apart from inconsistencies in temperature measurement.

G

If you have Maplins or a similar outlet for electronic bits and pieces, then seek out a temperature probe.
My Maplin one has a LCD readout in a 5 X 3 cm housing, which contains the small button type battery. The probe is about 5 mm diameter, 30mm long on the end of about 1.5 m of lead.
Provided you don't guillotine the lead with canopy, say, you can tape the probe somewhere on the outside.

I have used a portable electronic display with a thermocouple wire which can be passed through a door or window and taped into place in a suitable place on the airframe. Be careful in locating the probe. Don't place it in a area that can be influenced by engine exhaust heating and keep it clear of the airframe.
Of course if you are doing climb performance certification testing, you'll need to calibrate the setup.

I hope this helps.