Well, I lost the gas cap...

i was curious and i checked ebay etc for OP and there is no used fuel cap available for rocket 2.5 for now anywhere.
 
Pinwall cycle is worth a look, but given how long these bikes have been out it's not likely.

Crazy thought (most of mine are)- is this something that could be re-created with a 3D printer, assuming you could borrow one? If so, I guess how the plastic would react to fuel would be another concern.
 
This is now my 5th reason to BUT by or another modification must for the 2.5's. I wanted to buy one so bad but the 5 or 67 serious issues that either the dealer wont or are impossible to fix are a deal breaker, all of them make the bike unrideable. a non removable cap is a must for me, for this threads reason.
IMO there is no reason why they should have done away with the old style caps.
 
How does that foam react to gasoline? You don't want it melting into the fuel pump.
That's just used for a springy material to hold the Cometic gasket disc I cut out down over the hole. It was a one day solution and it worked great.