I would think about moving the bees into a Nuc.
A follower board will create more "dead space" for other vermin (spiders, roaches, SHB, etc.) to set up housekeeping in, whether you go all the way down or not, but I think by definition that a follower board seals off the unused area.
If you do decide to go with a follower, I would reduce the entrance so the bees can only enter their partition, and keep a close eye on the other side.
Have fun!