I have some mud flaps similar to what you are speaking of that slip over the draw bar. Mine were called Roadwings and made by a fellow in Redmond, Oregon. He sold out to someone else and they are distributing the mud flaps through Camping World.
When I purchased mine the fellow made 3 different sizes depending upon the width of the tow vehicle. Campng World just carries the largest size and at a very large price. I only paid $75.00, shipping included, for mine.
When I bought my trailer I thought mud flaps would be a good idea. I subsequently found that my Jeep Cherokee doesn't throw as much stuff onto the Scamp
as I thought it might. I can also feel wind drag when the mud flaps are on, so I haven't used them for a long time. I've found them annoying going in and out of driveways, they drag even though I've shorten them.
However, the mud flaps will be used this summer when I go to Alaska. Might be handy on the Alaskan Highways. I think I'll be glad I have them.