Don't use plastic period, especially cans not designed for gas. Fumes will bleed thru the pores of the container. New plastic can sold in Calif have a special liner on them (some type of chemical coating) to help prevent this. Also if you do open the antifreeze bottle, you'll probably get a pretty good spray of fuel squirting out. That type of soft plastic stretches under pressure just like a balloon.
Wedco (a Canadian company, I think) distributes the cans in the US, manufactured in Latvia (one of those eastern block countries). They will hold 60 psi without leaking, been thru fire testing, drop strength testing, salt spay corrision testing, mass lost test (amount of fuel lost thru steel or gasket). I know this because I have the original test reports from Australia and the testing lab that did the tests. I have no financial interest in this company and do not sell gas cans. They are just the best you can buy in the USA right now. Instead of plastic, you're just better off not storing extra fuel as suggested above.
George