Loctite 574 flange sealant is a possibility. Its main purpose is flange sealing in automotive applications, so it takes hot oil and pressure just fine. Surfaces must be clean etc.
Do a search - plenty of information on it on Loctite's site - and see if you think it is a possibility.
Cheap it isn't. 50cc (which is the minimum) of the stuff is twenty-odd quid.
Another possibility are bolts that include a sealing washer, like these
https://www.zago.com/products/sealing-screws/ . Again, not cheap, although I cannot recall how much (I used them last about four years ago)
Craig