Hi fishwater,
One extra thing you might try. This is something that was recommended to me since I also use sandboxes (see my system pictures). That is to make a few pieces of MDF or some of the poplar into strips, maybe 1 inch or 1 1/2 inch in width and glue them on edge to the bottom of the top shelf. This way they protrude down into the sand and act like sinks. Picture it like an upside-down heat sink. I'm in the middle of a couple of other projects so I haven't gotten around to trying it yet. It does make sense though.
Best of luck,
Dan
One extra thing you might try. This is something that was recommended to me since I also use sandboxes (see my system pictures). That is to make a few pieces of MDF or some of the poplar into strips, maybe 1 inch or 1 1/2 inch in width and glue them on edge to the bottom of the top shelf. This way they protrude down into the sand and act like sinks. Picture it like an upside-down heat sink. I'm in the middle of a couple of other projects so I haven't gotten around to trying it yet. It does make sense though.
Best of luck,
Dan