You know you dislike that empty class 3 hitch receiver on the back of your pickup. Oh, it's a great place to hang a couple of truck nuts of course, but perhaps you want to have a removable bumper sticker that says 'If you can read this you should probably be on the phone with your insurance agent.' or something pithy, or that landscape sticker or state oval, or ... You get the picture. Sure you can get an extra brake light module that fits there, or have someone weld up something and weld on a plasma cut sign that gives the world your opinion of that joker riding or driving way too close to your truck. I get that. But I have a 3d printer, and if you're here on this site, I'm guessing it's likely that you've got one too.
I've uploaded 4 variations, two of each, one in Step, the other an STL. Step may give you smoother curves, if your slicer supports it and your CAD program of choice should be able to edit it for remix. STL may be needed for some slicers. One of the two models will print with a flat back end, the other has a recessed pocket. Depending on your software you may be able to add custom artwork in the form of an svg file to add raised content in the recess, or to cut out that artwork from the flat face model. Step or STL should work across most CAD software. I can export to other formats if that seems needed.
Feel free to color match filament to your pickup paint job, or print in high viz to remind folks that they may want to be cautious. As most filament isn't expected to be UV resistant, you may also wish to paint for UV protection.There is about 2 inches between the end of the pipe and the edge of the hitch pin hole that you could store something handy for your trailer hitch, like a bit of grease to lubricate the receiver and reduce the likelihood that it will rust out just because you were towing your boat and got a bit too far back when launching or recovering said boat. I don't recommend keeping a spare key here, but you do you.