Planet Bike fenders are easy to find and have reasonably good coverage in 20".
SKS makes even better 20" fenders, but they aren't well distributed in the US and are hard to find. I bought a pair from a Moulton dealer. Here is a photo of those fenders:
The RANS 20" fenders (sold for their Rocket SWB) have tons of coverage and full wrap around the front wheel (you can cut them shorter if you want). The mounting hardware is designed for their bikes, but it can be modified to work with a folder. This shows them mounted on a Rocket with the front fender cut down to more normal dimensions:
Delta made my favorite 20" fenders, but stopped around 2000. It's too bad, they were inexpensive, light, and had good coverage:
