DerbyDad03 wrote:

Groove or on top of a batten? How is it attached to the sides? In
either case, how much do the sides extend below the bottom?

Grooves in the sides and front. The groves extend about 1/2" below the
bottom panel. The bottom panel is now kind of bowl shaped, sagging
left
to right as well as front to back.

Simplest fix is 3 screws through the bottom into the back. Put one in the
center first, then one more on each side halfway between center and side.
That should fix the sag in both directions; if not, slack off the last two,
add another one (two more total) between them and center, then tighten the
loosened one.