20ga Wave Washer/O Ring?

[Cyclones6]

I have a new 870 Express 20ga that I bought in August. I have done a few small upgrades and the latest is purchasing the GG&G follower. I decided to go with the unslotted one and ground down the slots in the mag tube in case I decide to add a mag extension in the future. Along with that I bought an older style spring retainer. The only problem is that since my express barrel is new it does not have a detent ball and the new style magazine cap does not tighten down as much as I would like, or as much as it did before these mods.

I have read that an easy solution is to purchase a special wave washer, but I cannot find one in 20ga. Also I have read maybe an O-ring. I am wondering if this would just be a normal rubber O-ring from the hardware store that I can match up to the size of the end of the mag tube? Maybe a lock washer of some sort?

Any advice on how I can make my magazine cap tight again would be much appreciated.

[Synchronizor]

Wave-style lockwashers work well, but I'm not aware of anyone who markets them for the 20ga. I'd suggest taking some measurements of your magazine tube, guide ring, and magazine cap, and looking for something compatible from fastener/hardware suppliers. If you can find it, I'd recommend going with a triple-wave washer over a single-wave, since it'll better support the magazine cap.