Square dowel with wood epoxy in the horizontal missing piece, and then, believe it or not, tooth picks with or without glue/epoxy to repair the threads in the hole!
The toothpick trick is probably the best single diy home life hack I've ever learned for repairing stripped holes in wood. If there isn't anything below the holes, take about 3 square toothpicks, tap them in wood epoxy, stick the in the hole and wait to dry, then break of the ends and rescrew. No need to drill or anything; the toothpicks will splinter and fill in gaps in the threads making a super tight new threadpack.