Is this something you set up to use just for the winter? if not, how do you keep robbers out during the season? Also what is "a migatory during the flow"?Put two cedar shingle shims under a migratory cover and one part way across the resulting space and you'll have a nice sized opening for winter (break the last shim off at whatever size youwant for the opening). Or put shims under the inner cover the same way. Or cut a notch in the "taller" side of an inner cover.
Here's a picture of one from a migratory during the flow. I've added a shim to reduce the entrance some for the winter. I don't have a picture of that: