Since the interior of a chest of drawers is rarely finished out well enough to suffice as appearance grade with the drawers missing, we have to do some work to make these areas look nice. There’s more than one way to skin this cat, but we usually use inexpensive ¼” plywood with one decent appearance face to line the inside areas and then trim out the joints with pieces robbed off of the chest. This approach has worked well for us.
The last bit before we move on to finishing is to make sure that the piece is structurally sound by re-gluing loose joints and otherwise strengthening the chest as needed.