I’m trying to find the best way to fill a hole in wood with a cavity behind it.

I’ve got a stair box which at some point in the past has had a chunk taken out of it that goes completely through the wood. Due to the fact it’s a stair box there’s a large hollow cavity behind the hole that means I can’t fill it with putty or epoxy without it easily being pushed through into the cavity.

What’s the best practice here?

EDIT: Attached Image

Thanks!