Unfortunately, there is no simple replacement.
Because the original lenses are no longer available, a couple of people have designed and are selling adapters that fit the opening and allow you to install a standard round flush-mount taillight (LED would be a nice upgrade). I believe I saw an aluminum version and another made of fiberglass.
A site search on "U-Haul taillight adaptor" might turn up a link, or a U-haul
owner might chime in.
As Bob says, you could get very lucky and find someone who installed the adapters and still has one good lens left.
For a more original appearance, some people install taillight units from 80's vintage GM pick-ups, but I believe some modification of the fiberglass is required. .
Welcome to the joys of vintage U-hauls!
(EDIT- I see Bob has already chimed in regarding the adapters.)