For small tubes I use a flat non-folded cardstock mount with 3 holes or slits - old Centuri style - and some Kevlar or good sewing elastic or contest rubber woven through the holes. 1/8" rubber is too fat for BT5. Watch out for cheapo elastic and rubber that fail quickly. The white Estes rubber is terrible because of the additives; trash it and use good stuff or Kevlar. Get a half pound of fantastic rubber for $17 from
FAI Model Supply and never buy shock cord again
. A few inches of elastic/rubber and then Kevlar gives some anti-zipper protection.
I glue the mount in with a very thin layer of 5-minute epoxy and use a dowel wrapped with plastic wrap to press it in place while it cures. It's up close to the top of the tube and will not burn through for a long time. If it ever fails it can be sanded out with the same dowel and replaced. I also like Chris M's tube-through-the-MMT-rings trick for larger models that aren't so weight sensitive. The cord will burn through faster but is trivially replaced.