My thoughts on splicing conductors.
1-Make a good mechanical connection, solder the connection if it is possible.
2-With Scotch Fill putty fill the voids in the splice to make a smooth surface to tape over. You could use self vulcanizing rubber tape.
3-Wrap the connection with varnished cambric or fiberglass tape, self adhesive is preferred.
4-Paint the wrapped splice with Scotch Kote
5-Put Shrink tubing over the wrapped splice, you may choose either Cold Shrink or Heat Shrink. Shrink the tubing.
6-Close up the job, providing mechanical protection to the conductors.
My advice is guaranteed to be worth exactly what you have paid me for it.