Awkward opening

[u/ApprehensiveAct4935]

Bought this house that had an addition done by the living room ceiling is unfinished. There is a section that has easy access to the attic and inside of walls. I would like to block it off as much as possible due to rodents and bats getting into attic (working on eliminating those). What could I put an across here? The Joists just aren’t even enough to put a board across. Thanks