When I built my trailer, I had to get a state patrol inspection for licensing purposes. One of the requirements was to have mud flaps if the axle centerline was below the frame of the trailer. Since I'm running 37" tall tires, it was most CERTAINLY in need of the flaps. And since it's a low budget build, I built my own.
I got a piece of 90 degree angle iron from Ace Hardware, wich cost me all of about 5 bucks
Miscelaneous hardware and fender washers to distribute the load.
Chevy logo floor mats which were really cheap from the local parts store (10 bucks for the pair)
Here's how it came out and it passed inspection