Needed to do some service on hydraulic leveling jacks on 5th wheel and needed to cap off lift side hydraulic ports. I know they sell flared brass caps at hardware store, but seemed like a waste of money at $4 a piece when each jack needs 4 caps. Mocked these up and they seem to work quite well plugging hydraulic fittings preventing dripping fluid everywhere.

Also grab the line plugs here:

Hydraulic Service Plug 1/4" Flare by ProjectSkyDroid - MakerWorld