in the new frame design thread, we had a discussion of many different ways to
make the top beam adjustable to handle different material thicknesses.
My preferred way (which I haven’t built yet) is to use unistrut 2484 brackets at
the top of the legs, and then have a short piece of unistrut across the top of
this (replacing the existing wood support brackets that are mounted at the top
of the legs) and then attach the top beam to the short pieces of unistrut
link to example bracket