I installed a fantastic vent cover over my fan-less vent, Very happy that I did. Plan to install one over the bath fan, and maybe over the other vent too, depending on which fan I install (may go with one of the Maxx Fan Deluxe, which has it's own integral cover.
It's a very easy install
One thing I'll point out....
not likely a huge big deal, but the fantastic cover is designed to allow for a full opening of the vent. The others, especially the cheaper ones limit the opening and therefore limit air flow a bit.