Probably no one else will ever need this but I'll put it out here just in case.
I found a source for the fuel filler cap (didn't try Radical to be honest).
Aircraft Spruce sells them. Search A36S Non-lock cap assy 6 hole and it should come up.
For my car, it is the Newton 300 which you can confirm with the dimensions given. It is made by Newton. I'm not yet sure about what Flange/Neck type yet but I'm sure someone that still has their cap can chime in and tell me (which I would really appreciate!).
Like I said, probably no one will need this but the system was sold off of my car in the first place so someone wanted it (perhaps converting away from Dry-break?)
Joe,
I purchased a spare from Stradale thinking myself or someone else will forget to latch the cap correctly and we'll lose it. I don't think the spare was as expensive as the link you have there.
It might just be easier to give them a call knowing you'll have the correct part.
