Why not make your own? I found this photo on Pinterest and it looks easy enough to do, make it out of strips of 1/4" Baltic Birch plywood. Not a lot of detail shown, but should be easy enough to figure out. I would guess that the center piece at the ends needs to project slightly to provide a "key" to keep the sections aligned where they intersect, and I would have extra sections to slip in between the layers when cutting a large recess to prevent deflection of the plywood and an unintentional deepening of the recess.