I think the easiest,quickest way, might be to just screw it together, with drywall type screws, or those special ones for MDF, if using MDF, then filling seams, ans screw head, with light weight bondo, then sand, prime and paint.
The bondo tries very fast.
I have a rental house, where the tenant had put holes in 5 hollow core doors. Instead of buying new door, and having to hang them, I pulled the caved in holes out, putting a piece of ply inside, behind the damage, then did the bondo thing. Fast and inexpensive.