I confess, I finally gave up and cut them. Couldn't tell once installed. Probably overkill, but I used contact cement to glue the halves to both sides of the mounting plate. I think the normal compression of the screws would have been enough to keep them in.
I am disappointed the supplier did not test them first to see how difficult they were to install. I think they used too hard a compound to make them, something like they make the sway bar rubber grommets. The originals wiper motor grommets were much softer and certainly could be inserted in the holes.
I will notify the supplier of the issue and hope they will improve the next batch.