I finally got around to wanting to make some but had nothing close to the underlayment he used but I have boxes of 1/8" scraps so I recreated it in Fusion and made the slots where the thickness is adjustable.
I’ve worked out the numbers for the medium and the 1/8" Baltic Birch from woodpeckers and included them in the zip file as well as the Fusion 360 file for any other 1/8 class materials you might want to use. corner clamps.zip (65.4 KB)
When using the Fusion file you will want to go several thousandths less than what you measure as that is the only kerf compensation.
Thanx again to @paul4 for the original design.