Ok here is a quote from Ernie Buda the President of the Sacramento Beekeeping Association. I took this workshop with him and made the follower boards.
"Annette, as it where I had some very expensive 3/4 inch plywood "scraps" that I received at no cost and that is what we used. I have made others of common 3/4 inch plywood. I have recently made several sets of medium (I use all medium supers on my hives) follower boards there where 1/2 inch thick plywood. I would believe that the bees don't care how thick the material is. They seem to accept the inside surface as the outside of their nest. They do congregate in the space between the hive box and the follower board but I have not had any build comb there. The follower board is exactly the same size as the frames except for the thickness and of course it is solid. Some members have told me they made follower boards by using a standard frame filling in the center with solid material. I am not sure how that works out, I would speculate that the bees would spend a bit of time on filling any little gaps with propolis -- a waste of bee resource -- if my speculation is accurate.
I have seen one members follower boards and they made contact with the end of the supers -- eliminating bee space at the end and I suspect making them very hard to move -- I am sure the bees put propolis where the board contacted the hive body.
I would suspect that any solid material that will stay flat in the hive atmosphere would work. I would avoid metal because I think it might act as a conveyor of heat. The airspace provided and the, in general, low conductivity of wood are additional positive attributes of follower boards. The top bar because of strength issues should not be plywood, in my opinion. Any solid wood would work, attention should be paid the wood grain direction to lessen the probability of the top bar splitting.
The follower board is basically easy to make if of course you have the equipment. The only thing I have found to be of importance other than just cutting the pieces the correct size is that the top bar needs to be the same thickness of the board material. Care should be taken to sand the top board so the surface is flush / there should be no edge, even a small one for the hive tool to catch on when you clean bit of wax off.
If any one has additional question about follower boards they are free to email me.
OK that is some information about follower boards. If anyone wants to email him, let me know.