The flex duct can be cut and then reconnected using the proper diameter "coupling". A local sheet metal shop can make a coupling for you if you give them the diameter. It will be a round sleeve about 6" long with at least 1 raised bead in the center to add rigidity, but it really works well for wiremold flex to have 3 beads. 1 bead in about 1-1/2 inches from each end will give the panduit strap a place to rest without being pulled off.
Panduit strap is like a large zip tie that is applied using a special tool that pulls it very tight around the flex and cuts it off at the same time.
You can use screws to fasten to the collar, but I wouldn't recommend it.
"The physician can bury his mistakes, but the architect can only advise his clients to plant vines." -- Frank Lloyd Wright