We have not done an extensive test of fat bike fenders yet. However, we have gone through about a dozen fender options and landed on this set up below. We find it keeps about 90% of the mud and water off us. It's not as effective as "rainbow style" fenders that cover half the wheel and keep about 99% of the water and mud off you. However, we have not found that style fender that works for fat tires and is durable.
Our solution comes in three parts:1 - Portland Design Works Dave's Mud Shovel - Front
2 - Portland Design Works Dave's Mud Shovel - Rear
The combo runs about $50-60. Installation is easy and takes about 10 minutes. No tools required but scissors are useful for trimming zip ties.
We recement ordered the SKS models which look similar. We will report back shortly on their performance compared to the Mud Shovels.