Also study this:
And maybe think about rigging up a vacuum chamber (an old fridge compressor and a re-purposed pressure cooker might do the job)
Used to doing this on a bigger scale, vacuum bagging of epoxy layups for nautical and motorsport applications.
But, fundamentally choose a properly designed material system that you can source, experiment, and practice with, until you understand it.
Don’t expect to get it right first time. Dolloping or injecting things, one-off, with a glob of random gloop is easy. Doing it properly, for engineering reasons, then evaluating the result scientifically, rather than because “it seemed like a good idea” is quite different.
I say again, it’s not generally done for thermal reasons. It’s an environmental protection, ruggedness, none-maintainability, built in obsolescence, solution, which is perfect for those markets. And also costs quite a lot to do properly with decent materials, and hard-learned experience.
If you make e.g. P60 drop-ins for real tactical (or wannabee) use, on e.g .308 upwards, then yes, it certainly has a use. And you’ll never know whether you got it right until, say ten years of field feedback, or maybe 168 hours in the environmental simulation chambers.
Otherwise good construction, use of none-pc solders, pristine cleaning (just a bristle brush and 50:50 IPA/Di water), and suitable conformal coating, with maybe some re-enforcement of weak points e.g. USB connectors (ugh), should be enough.
