I just finished the rocket launcher (I made the gray end) and started the base.
How build a ring with DURO putty (Green Stuff)
Make a thin* wire with a small ball of putty. Use a flat tool like a ruler, a lot of water 'cause the DURO is very sticky (!!!) (*) The only difficulty of this tip is to get a perfect wire, with the same diameter everywhere... Wind the wet wire around a cylinder as a spring. I use an old stylus ^_^ with its cone shape I can make different diameters and be sure to find the good one! When dry, I cut straight all the rings. And I close the good one with Super Glue.