The redneck in me says just duct-tape it, but ....
I found (and used) a simple plastic replacement tab made just for license plates. I think it was $4-5 at an RV parts store.
You can find suitable sheets in places like Home Depot. You wouldn't even have to cut "tabs". You'd just need to drill holes for the license and for the mounting screws onto the trailer.
The tabs on my '96 Towlite were broken off when I got it, I just cut the stubs off even with the tail light and screwed the license plate straight into the trailer skin with some rtv to seal the screws, have never had a problem and you can't tell anything is out of the ordinary.