Here is how I modified four of my hold down clamps so I can clamp thick materials:
I used four 100mm M5 threaded rods (I couldn’t find any M5 bolts or screws long enough), four small fender washers and four M5 nylon locking nuts.
Then I put them all together like this:
It’s a much stronger clamp, which can be very important when pushing a Dewalt DWP611 Spindle through thick hardwood.