Guys that ride the dunes: banshees weren't made for mud buy a 4x4.
Ok now ill tell you what I think cause I also ride in muddy wet places. I cut every bit of plastic off my stock airbox lid that I could but still be able to use the clamps and put an Outerwear on. To tell you my opinion it works "ok" as far as keeping mud out but its far from water proof. That being said that box on eBay looks sweet. Get it, try it , and let me know how I works!