OK thanks. I might try tacking a bit of ply to an empty frame.
Personally, I wouldn't. I have a lot of other insects like Palmetto bugs, spiders, ants, earwhigs etc, not to mention SHBs that would set-up shop in the unused area if the bees cannot or will not patrol it. Then there are the lizards always looking for a slow bee or other bug.
I would add frames with foundation, mix them in near the center and feed 2:1 to help them draw out the frames faster.
Just a thought, have fun!