This cap is designed with the intention to prevent contamination as well as act as a seal to prevent unwanted fuel spills during transport.  

I've tried this part on a 5 gallon can as well as a 2 and a half gallon can. Universal compatibility is yet to be determined, but in the case that this part does not fit your particular can, I've added a SolidWorks file for your convenience. 

This part was printed with Polymaker PA6-GF Nylon filament @ 0.2mm layer height. I would not recommend ABS, as the chemical compatibility with gasoline is very poor.

Feedback is greatly appreciated!