I decided to try a follower board one day while I was out in the field and didn't have one. But I did have a cardboard box. I cut a big section of cardboard and held it against the end of the TBH to get the angles right as I bent over the edges. Then I put it in the hive with the bent-over flaps holding it in place. Worked great. A few bees crawled under it sometimes and got into the end space, but they didn't seem to stay long. After almost a year, I finally pulled it out because a. the cardboard was getting warped from being moist on one side, and b. the hive had grown to where it didn't need a follower anymore.
One surprise: the bees didn't chew on the cardboard very much.
"Before I speak, I have something I'd like to say. . . . I will try to keep this short as long as I can." Yogi Berra