It fits just fine. It is a bit thinner compared to the original. But the key works and it's fine anyway, with the cap closed it doesn't show. It was a necessary purchase because they were stealing my gasoline on the e36.
I would prefer a design closer to the original cap look, even if it had a longer key that goes inside. But that's not the store's fault, it's the manufacturer's. And the truth is, even when I searched abroad, I didn't find anything; only in Greece was there something like this, which definitely does what I want. The only thing I'm thinking is to paint it another color and write that it takes gasoline, to avoid mistakes at gas stations with mixing.