Thanks for sharing / posting.
So does this mean that Rotopax has redesigned/updated their product? (in the time since you bought yours?) Such that the exact 2-gallon Rotopax container you have isn't being manufactured anymore? (Hence the caps that they send you aren't the same spec/design as the one you need?)
If the product has indeed been updated/redesigned in some way, then it isn't entirely unexpected for them to stop carrying replacement parts at some point for the older version. (Though when you call them, they should ideally still be knowledgeable about the parts you need....)
Sorry to hear about your runaround!
'95 Ford E250 RB30 PH