I know this post is old, but I've gone through that little protection tube problem today. Indeed, Drennan does not seem to be able to provide one of those simple plastic tubes that we get with ANY feeder rod (really, is that too expensive to them?); so I've met the missing requirement as follows.
1) A Round Conduit Black 20mm x 2m from Wickes (electricals department) - £1.15. Another option was a "push fit" waste pipe 32mm x 3m (£2.05). The cable conduit is slightly flexible, but since it is to be cut at a length of say 60 cm max, the cut will still be very stiff. The waste pipe is stiffer but 3 m was a bit long to carry back home walking. For the Drennan's tips, I needed a section of 65 cm.
2) A stop end that I already had. Otherwise, if selecting the waste pipe then a stop end that match the tube can be found for £0.99. On the other end, a piece of glass photo print paper and strong cellotape completed the build.
The result is good enough for £2 and will make other tubes if necessary.
It is true that my Nash Scope OPS Recon Rucksack nows looks like an Argos Need-A-Loo Excel Portable Camping Toilet, but who cares? When crossing any street in Oxford, I've always been an alien!
