Ditch that plastic block and the one that is likely in the other corner.
Put glue on all the joints. Push back together and nail. Clamp if required to hold together.
Glue and clamp wooden blocks in place of the plastic. Nail where they won't be visible on the face.
Put glue on all the joints. Push back together and nail. Clamp if required to hold together.
Glue and clamp wooden blocks in place of the plastic. Nail where they won't be visible on the face.