Fixing old dulcimer

New to the forum! I was just given a mountain dulcimer by Samzie Adams, made in 1972. A friend pulled it out of a junk pile on the floor of his shop and handed it to me. It is in great shape, except once I strung and played it, the top has collapsed a bit. I am super handy and am wondering how I can shore the top back to level. I have thought of putting a couple posts inside, but it doesn’t look easy. The sound holes only allow a pinky to go inside. Wondered about getting posts underneath the fretboard. It has a channel inside. And seems safer than raising the sound board, since it is old and has a couple cracks already. Have thought about putting a couple holes in the fretboard and slipping some kind of support under the fretboard. Have thought about a slot in the side to insert a support. Or should I just leave it alone. I don’t want it to get worse. I can be “adventurous” (not always a good thing)when it comes to DYI !