Joining cable

[Thebeeman]

Make sure it is adhesive lined heat shrink as it's sturdier and waterproof.

Plus insert a length of welding rod or similar inside the tube to create a rigid section over the joint.

[knighty]

as above, adhesive heat shrink - I like the welding rod

but also.... layer up the heat shrink, 3 or 4 layers, and make then longer than needed... each bit 2inch longer than the bit under it

if it got bashed/broken somehow and needs protection, sleeve it inside a bit of water pipe? a roll of 15mm pex tubing is cheap enough

[AE-NMidlands]

as above, adhesive heat shrink - I like the welding rod

I don't - not if it's like a "stick" welding rod...
The coatings are horribly corrosive if they get damp and should never be held in the hand for any length of time. They are full of fluorides and other nasty stuff designed to instantly etch the surfaces you are welding as well as make a protective oxygen-excluding cloud.

Almost any stiff wire or plastic rod would be better as long as it didn't have sharp ends which could chafe the insulation on the conductors.

[knighty]

good point, my brain automatically went to tig rod without giving it much thought!

could always bash the flux of a stick welder rod?