Nice. Does a standard ball hitch reach through the 4-inch tube depth so pin can lock it in?
By the way, 4X8X.25? What are you planning to tow, a house!!!! Just kidding.
I do like the design's function. I think I would have added two more receivers close to ends (one each end) to attach hitch carrier. Using two adds a lot of stability, and your design would make it very easy.
I love my Curt hitch and my GoRhino step. Not super duty but then I do not plan on towing anything large so this is fine. Both readily available on Amazon. The step is large enough to use with one door open and it is shaped in such a way to avoid those insanely painful shin versus trailer hitch moments.