I would think that the retainer ring would be the best solution, like the one in siltys post, i have found these in a couple of vintage watches and seam to do the job well, the problem here is finding one that would fit?? the original being to large and you want to use it to put the other flytimer back together. If you had another that was either too large or too small you could split it on the oporsit side to the crown and puchers to alown it to shrink or grow to match the circumference of the movement and the case back presure will then hold it in place maybe