Doesn't really need to be over engineered. Heat shrinkable tubing comes in all sizes and flavors.
It starts out rather big so you can slip it over your splice. When you heat it, it shrinks quite a bit.
A Bic, butane torch. That'll do. Your local hardware store will have assortments.
There is even glue lined heat shrink. So when you heat it and shrink it. Hot glue oozes out. Makes even a stronger splice. Not 100% necessary.
If you don't have a soldering iron and solder (not acid core plumbers solder....rosin core) then the Western Union splice will be the strongest. When the heat shrink is.....shrunk. It will further reinforce the splice.
Stagger splice is the way to go. Just make sure you have enough exposed insulated wire to cut and slip a piece of shrink over it before. Kind of a no-brainer. Try to just nick the insulation when you strip it so you don't cut any inner strands. If you do. Just a few strands. No worries. For low voltages, the 2.5" minimum spacing can be decreased quite a bit.
Try to make the splices no bigger than the insulation. And neat. It ain't that hard. Get the cable up off of the floor in a comfortable space. Since flame will be used. Maybe put the liquor away for a bit. A bit of levity.
And yeah. Red goes to red, black to black, green to.....you get it.
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