The flat gaskets that come with the FF work fine, but you have to tighten it down so tight to get a good seal, that it is impossible to get the cover back off.
What I use is one foam ring from dollar store foam that can be found in the craft section and the flat silicon gasket that comes with the FF. This what when you cinch it down, the foam together with the silicon seals it, but the foam ring keeps it so you can actually get the cover back off. Plus the foam ring cut slightly bigger, keeps the silicon ring in place.
The hollow one looks good, but it is still silicon on plastic, which means it has to be cinched down tight, which makes it hard to get back off.