Hi there, it seems you haven’t had a answer yet!
Two things come into play here. the first is the specifications of the crystal reference you are using, some are very stable with quite high temperature variations, others have less tolerance.
The circuit you put the crystal in will also be a major variable depending on the stability of the components. Particularly any tuning capacitors as these tend to not like anything much above 30 degrees C. It is possible to obtain components with high temperature stability but they tend to cost a lot more.
One solution is to mount the devices in a box that is also a heat-sink and use a small cooling fan.
Unless of course you are planning a voyage to the sun, in which case it’s all going to get toasted!