Just to add my tuppence worth, I've made a couple of box mags in the past, a long time back lol, & yeah a lo or mid cap, as well as your donor drum/box mag, when it came to getting a good feed tube from one 't tother, I found a 7mm expansion spring to be perfect, they're available in many lengths, once its in the position you want, stick some heat shrink over it, warm it & it will keep that position, resisting knocks & even disassembly.
simples๐