On most domestics you always use the same section as the base. The green thing in the photos is adjustable. The bit inside changes size when you twist the end. So if you add another section, just pop it out, and fit it in the new base section. For 90% of domestics its always in the same section so you can stand the pole on the ground without damaging the end. 6 quid from the tackle shop (see, fishermen have the same problem).
The thread is fitted like this:(I do wish i had taken photos), The no3 section I cut down where it was wide enough to fit the thread in loosely, with the bit left over I cut 2 4inch sections. Into the narrowest, I put the thread(I had to shave it a bit). Glued it. Then fitted that into the other 4 inch section GLUE. Then taking the thread now surrounded by double thickness carbon, I glued that into the now shorter NO3 section. So where the thread protrudes, in the photo, There is actually treble thickness around it. Its tough as nails.
I didn't actually measure any of it and did it by fluke. Dangerous. Measure it properly or you could waste the section.
Doing the downs is hard with just the section with the brush because its thin (trust me) But with 2 sections its fine, if a little long. I can carry the down pole in the bag as well so it's not an issue.