I build my own migratory type top/bottom boards. The same board works for either position. Works great since I keep hives up on stands, not on the ground. An 3/4" piece of A/C grade plywood, with 3/4 x 3/4 inch pieces glued and screwed on the two long edges and one short edge, and 2 coats of paint both sides. This board can be a bottom board entrance, or a top board entrance, or a top board no entrance. Works great for me. No inner cover, the bees glue it down so no wind blows it off. Slip hive tool under and pry to remove.
As an added bonus, you can stack a hive on top of another with this board very easily, the top board of one hive becomes the bottom board of the hive on top.