Do NOT put a foam cap over the OCXO. It's designed to expect a certain amount of heat to escape. If you prevent that, the unit heats up to a point where the oven throttles back, but the temperature keeps going up! The oven turns off completely, which means that there's no regulation of the temperature, but the temperature keeps rising just from the normal operating power of the oscillator! Worst case, the temperature keeps going up until the circuit fries. Best case is that the unit operates above its design temperature and the performance suffers. The oven controller could even start oscillating which would really mess up the stability!
Most OCXOs operate their ovens in the 50C to 80C range. Each unit has its own sweet spot for temperature. The controller will try to maintain the temperature within a very tight range. Very high performance units might have a range of 0.001C. Maximum operating temperature is typically something like 10C below the oven temperature.
Protect the OCXO from drafts (hot or cold), sunbeams, etc., and leave some ventilation room around the outside of your GPSDO, but otherwise, leave it alone. I see that the NVG47B oscillator is available in three operating temperature values from 50C to 85C. Unfortunately, there's no easy way to tell what the maximum operating temperature of the NVG47A oscillator in the TruePosition is. But 40C near the oscillator shouldn't be a problem. Temperatures over 50C would require some careful investigation.
Ed