Petey, unfortunately they are designed to "clamp" 2 x 19mm stock... can even handle that recalcitrant 3/4" stock as well. They have a reach limit of 30-40mm.
Below that limit, I use a range of spacers cut from 6mm thick MDF to pack out underneath or some 5mm thick "up market" acrylic spacers, but being clear... too easy to lose only to find when no longer required.
In your case, you would need to counter bore the benchtop underneath the dog holes to achieve the desired thickness, Shouldn't affect the integrity of the benchtop.
If your first cut is too short... Take the second cut from the longer end... LBD
