Battery relocated in trunk.

to attach the cable lugs, I have used solder pellets
Solder Pellets, Slugs and Flux | Waytek Wire

put the lug in a vise, drop the pellet in and heat the lug to the solder pellet melts, have the cable close by and ready, end stripped, fluxed,,,,
plunge the stripped end of the cable into the molten solder in the lug.

I have also crimped the lug in a vise using a crimping die, I flow solder into these after crimping.