That's one way to do it, I suppose. I would go to the junk yard and pull one, or shave down the tapered plug you mentioned to have more clearance, or just get a price of plexy glass cut it round and rtv the edge of that. I personally would rather have plastic only around it, not sure if any could be had but what damage could happen if it came loose? Plastic will not cause the damage a metal washer would. Or if the job weld broke off and into the bell housing? Just thoughts.
2001 ZX3 2.0L Zetec-
Hatch Nation #42
Going through Rebuild See "The TICK Build Thread" for details (http://www.focusfanatics.com/forum/showthread.php?t=289168&referrerid=62897)