Simple design made from scratch to learn Tinkercad.  Note the rounded corners!  Not as impressive as the hex-hole pattern I did eventually get to work, but that had too many crevices, especially in slightly drooping overhangs.  I may end up sanding the inside that sits against the sponge.

It's not quite the same on each side – though that might be a future improvement.  One side has 2x 90° angles.  The other side has 2x 92° angles to better fit our sink, but should be slightly wider still.