Putting a plywood sheet under your mattress is an option to use as an alternative to a bunkie board. This could help with the sagging issue as long as the plywood sheet matches the size of your mattress. If you’re using a box spring or foundation, you can lay plywood under your mattress to make your sleeping surface even sturdier. The extra layer can stop the mattress from sliding and can help reduce the squeakiness issue with older box springs.