Even a TCXO is not stable within the LNB!
And there is a lack of space in the LNB, it needs a lot of soldering skills to get this done.
Preferred solution is to feed the 25MHz via cable, preferred LNB has min. two outputs, use one of these to feed the 25MHz. I use a ferrite bead as a transformer as suggested somewhere in this forum.
TCXO or later GPSDO can be placed in temperated shack