[tsubaki]

When you get the wood out the way, take a rod and bend the end to have about a 2" 90 degree angle in it.
Probing in the bottom and any other places, see if there are voids toward the front or sides. Making the angle slightly longer than the fill area makes it quicker to find holes or gaps.
If you do find voids, you can pack some stuff in that general area to slow the hemorrhage of the fill material.
One thing that worked for me was a scuff material designed for floor sanding (from HomeDepot or Lowes). Looks like woven glass. If I can access the site long enough I'll try to see if there are pictures of the stuff (so far unsuccessful).