If you plan on them coming apart again, then correct size wire nut wrapped with tape so it doesn't come loose, which will cause it to burn though.

Connections that loosen can reallly drive amp loads up, I believe.

If you are planning on connections being somewhat permanent then I'd first slide a piece of good shrink tubing over the wire, then solder them together, then slide that shrink tubing up over the now soldered joint and shrink it in place.