Jim,
I did something similar. I picked up a FCD for the WSM case cheaply, and shortened the bottom of the sliding part for use on the RSAUM case. It worked perfectly. I haven't yet tried the short die/washer thing... Yet. I think it should work, but it seems like it would be extreemly S - L - O - W. Where possible, I'll stick to shortening the collet, or bite the bullet, and spring for the $25 custom FCD. It won't break the bank, but is still a PITA.