What I have been using for some time now, instead of classic thread lock, is just a tiny dab of regular plumbing thread paste, the liquid version of plumbing Teflon tape. Just a little bit on the top one or two threads before screwing the cap down, wipe off any excess once the cap is on.
The top cap will still be definitely removable, but it has enough grip so it doesn't unscrew with standard carto changes.
The loktite is easy to overdo, the Teflon thread paste is more forgiving.
Cheap, readily available.
The bps are certainly alive and well, they are absolutely stellar pv