Thanks. I found the right part #, which I suppose I should post. For the cap ring it is: Silicone $6.11/100 - 9396K18 with the disclaimer that it's not a square sided ring, but that it works.
That was easy, but what about a McMaster-Carr part number o-ring for the plastic firestone gas dip tube?
For the plastic dip tubes I simply pulled them and replaced with a standard diptube o-ring and a stainless washer. Works perfectly.
I also did one keg with two o-rings stacked, no washer. That holds, too. Same o-rings I use on all my kegs, bought in bulk from McMaster.