Just putting disc brakes onto a frame with no guides. I can use a couple of the old wire brake guides but towards the back of the frame there’s nothing. I’ve seen a few expensive zip tie on guides but there must be something better or cheaper (rather than just a zip tie around the frame)? Any ideas?

Something like these?
I recently got hold of an old Jack Flash for messing about on and thought about these when installing some disks, I then thought, nah. I stuck with cable ties for rustic authenticity (and the fact that I always have a bag of them knocking around the garage)

Can you drill out the existing guides to fit a hose through?

There’s a voodoo way of loop-de-looping a zip tie so that it holds a cable nicely rather than just wrapping it against the frame. You use a washer or similar, pass the zip tie through it, around the cable then back through the washer and around the frame.