The DIY charcoal canister that I build (modeled after Edwardb's) is the same size as yours and seems to work fine at keeping gas odors out of the garage so I don't think the problem is the size of the canister. Start with checking the filler neck seal at the tank. Did you install the one that FFR provides or replace it with the Ford OEM part? The generic ones that FFR provides have been known to tear if you look at them funny.