The Bike Friday fenders are neat because each attaches with only a single bolt. (No stays going to the dropouts). I haven't put mine onto my new NWT yet, but they look great!

On my old folder (a Dahon), I used Planet Bike Hardcore Recumbent fenders. After taking a dremel to the stays to get everything to fit correctly, they worked great. I'm thinking that the stays are made long to fit 451 wheels, but that's just a guess. They run about $20 for the rear and $15 for the front.

In either case, you'll have to remove the front fender for packing. It may be possible to leave the rear one on, as the rear wheel isn't removed from the frame either. It'd be really tight, though.