From the site listed above:
I'm sure the tail lens on the below page, isn't original, but the curve of it, reminds me of the tail lenses on Renee's 73 Chevy P/U. Would be very easy to cut one down to fit with a coping saw. You might try it.... try to get a used one to find out. You can probably find a cracked one at a junk yard for almost nothing.
Then, if it will work buy new, and use the old one as a template. Or if it comes close, but is too large, you might trim the fiberglass a little larger and use the whole assembly... graft it to fit, and inherit backup lights
on your U-Haul... in the process. Looks close guys! Hope it will work for you!