not seen a one, but i do agree they are a water and mud trap
the problem is tho if you make one were would it be mounted without causeing water/ rust ingress along with possibility of trapping water behind it?
since they like to rust around the rear acher point and rear inner arch were it joins the boot floor then onto is he the rear vent and rear inner sill
one of the biggest problems is the shultz on some cars (i found coil packs been the worse) for holding water and rotting the body
going off some of mine ive had best maybe enlarging the fuel pipe cover and extending it to cover the inner sill and floor and making a rear section to cover the rear vent and boot floorr along with cleaning off the dirt and shultz then treating and resealing ,or alot of redlead and seam sealer lol
im just going off older nissans some used rubber aswell which did help but could hold dirt on to the chassis leg
also noticed some of the newer cars that have a lot of covers seem to hide dirt and water and in some cases rust