The standard inner cover for a Langstroth hive consists of a board of some sort with a frame around it that observes bee space. There is a handhold/ventilation space normally in the middle of the cover.
One really simple option for an inner cover is to just cut a sheet of fabric!! You can even buy them precut here. I find these work really well – I used to use an old 2 WW military tent that got destroyed by a wild animal.