As long as you can’t use a U/L listed extension cord, it’s just a simple power cable. Back before there were ‘short’ versions, I’d made several myself, by ordering a new cable, wanting to keep the original, and cutting it down to accept a store bought wall plug. Normal power care involved, but it really is just a power cable. I’d be a lot more concerned about jury rigging a battery to supply the correct power to a non-battery equipped speaker.
These days, ordering one off of Amazon that’s close to the correct length is much easier, prettier when I look at it, and provides me a place to point fingers, when one fails….but after years, neither my own, nor purchased ones have. Like I say, it’s just electricity.
The only concern I’d have is I don’t know what ‘twin core’ cable is. If it isn’t standard power cable, don’t use it. Especially if it’s designed for speaker use (much lower voltage) or video use (again, substantially less power transmission). Use regular, every day, power cable. Available everywhere.