I wouldn’t use anything in that cavity—there’s no reason to. The original setup had a fine wire spring going from the coil tabs to the plug wire and it was encased in a thin glass tube. The tar kept things aligned and tight in there and isolated any moisture because there was not a soldered connection to hold the plug wire spring in place. On my engines I completely clean out the tar and I make small inserts out of delrin that go in the plug wire opening in the mag plate that have a tiny hole in the end and a "collar" on the end of the insert that prevents it from going in too far. It is designed such that the bakelite insulator, when screwed in, pushes the insert towards the coil tab. On the insert where the tiny hole is, the stripped plug wire where it protrudes, is cut and soldered in a ball shape so that when you screw the insulator in it pushes that solder ball into the coil’s tab. The wire cannot be pulled out due to the solder, and rotating the wire won’t damage the coil’s tab. It is clean and works well.