Probably the best way to go is to use the 24g UPOCC copper wire in teflon insulation, the stuff I sell is solid core, not stranded, I'll have to fix that mistake on my site... The Neotech premade cable is pretty good but stranded wire and PVC insulation won't be quite as good. With the 24g wire you need to combine it into whatever finished gauge you want, I'd suggest twisting or braiding to make up a larger gauge wire, then combining 4 runs of that wire using a simple 4-strand braid. If you do this you will end up with close to the best copper cable possible. UPOCC silver or the silver/gold alloy I sell is better but the price gets up there really quick. It's worth it for interconnect cables but maybe not for speaker cables, just depends on what you're going for.
The UPOCC copper litz wire I sell can make for good speaker cable but it's not the best match for some speakers, if it does work for your application it's better than conventional wire. I tend to recommend the litz wire for internal speaker wire, between xo and drivers and as main speaker cable for higher efficiency speakers. It always works with single driver (no crossover) and speakers with 1st order xos. Multi-way lower efficiency speakers can be a bit of a crapshoot.
For lower priced cables I'd look at the surplus mil-spec wire from Apex Jr. In some systems it can be a bit tizzy or splashy due to the silver plating but in other systems it's fine. Speaker cables are much more variable vs IC cables as far as compatibility and performance in different systems as the electrical characteristics differ much more vs what an IC cable sees.
Also, you'd need 32 runs of the 24g wire to make up a 12g speaker cable, half that for 15 gauge... the 12g cable is a LOT of labor to put together!