FWIW there are a bunch of bare modules and eval board clone versions of the CTI OSC5A2B0 for crazy cheap.
Best ebay search string is "OCXO 10MHz Frequency Module Crystal Oscillator Constant Temperature" for the $20 plug and play versions and "OSC5A2B0" for the $3 bare modules. I bought 5 of the former and 10 of the latter.
The two eval board versions that I've tested hold 0.1 ppb with some scrap bath towel to mitigate drafts. Limit of my GPSDO is around 0.001 ppb best case. So we are talking very accurate frequency references for $20 delivered.
They arrived set within 1-2 ppm, but a bit of tweaking using a GPSDO and scope got them to 0.1 ppb. I'm going to mount one in a case withnone, just insulation and another in a case with active temperature control and then monitor all 3 long term.