Excellent, rigid under-mattress base can add firmness to a sagging foundation.
Can be placed on a platform-type base - the slats are wood and could potentially be adjusted in width to adapt to the base.
This could also be positioned between mattress and box spring for added firmness when needed. (It could stay there but it could be removed easily as well and so is a great solution to a guest bedroom where the need for firmness may vary.)