I've been wanting to come up with something. Visibility to the right rear is pretty well blocked by the canvas enclosure.
First I tried a mirror that would use the existing door hinge bolts on the CJ-5, but it would not work on the 3a, the door hinges are spot welded, so no existing bolt holes.
I don't normally use the doors, even in winter, as it just makes the jeep a little too confining and everything fogs up anyway.
The Model A Ford vendors offer an exterior mirror that attaches to the door hinge, and that got me thinking.
So this is the result. I just bought a mild steel 3/8" rod and bent it to shape using a MAPP torch, threaded a portion of it, then I heated and flattened the end, drilled it and attached the mirror. The leather washers help hold it in place while still allowing it to fold, as I manage to walk into it pretty regular!
I need a convex mirror but for now this will do. And it works on both the CJ-3a and the CJ-5.