Yes, I found pictures of someone who just used a board to fill the gap as well.

Noticed that underneath this gap there are some holes (4 or 5) in the fiberglass that appear to be made to bolt this piece down.

Another question I have- should this "gap" piece be secured to the back of the rear seat frame so, when the gap piece is bolted down, the rear seat is secured in place as well?
This is the only way I can see to secure the rear bench in place, unless its just meant to sit there unsecured (seems like it will be heavy and tight enough to not have to worry about it moving).