Designed (with a computer) by Bill Cutler and Frans de Vreugd 2001, level 13
(Hard Maple and Jatoba, 2.25 inches)
Two 1x4x6 pieces in each dimension. Making top dark wood and the bottom light wood makes the level 13 solution unique. The number of "holes" (internal voids) is 13 because the volume of assembled shape if it was solid is 104, and adding up the volume of the six pieces gives 91. Here are sheets that came with the puzzle (copyright by and courtesy of Bill Cutler).