Acebird. Greetings Brian. I am pretty sure that a follow board is not meant to completely seal off the back half of a long hive. They are 4 foot long boxes, with inevitable slight warping of the wood, so the follow board might be tight in one location but looser further back. Also, it is easier to seal a square box with a flat board of top and a weight, than a vertical board inside a box.
The follow board, as I understand it, is to create a false back, restricting the building of comb to the front. Since it is almost impossible to make it truly sealed, most people leave a 1/2 inch gap at the bottom, so bees that get through (and they will) can get back in again.
Ben. My bad back laid me out if work for a week not long before I decided to beekeep. That alone set my feet upon horizontal beekeeping. I have three of them as of now. It is like opening a file cabinet, and shifting around the files.