The squeegee holder (the green part) is composed of 2 plates of very hard plastic (polypropylene probably), you can use hardwood or plywood with no problem i think.
I drilled the center axis and inserted 2 bearings : they act like a spacer that let the piece move freely, and will let the squeeggee lay flat on the screen even if the arm is tilted down or up a slightly.
Then, with a circular saw i cut 2 slot for the 2 tiny clamps destined to hold the squeegee. I angled the bottom of the slot with a rotary tool to provide more angular movement.
I drilled in the side of the metal clamp and screw them in place : but able to move freely since they will define the angle of the squeegee during the print.
It is important to place them so that when they clamp the squeegee, the squeegee is centered. Otherwise the short clamp can not hold or the squeegee can bump into the U shape slider assembly.
Then to be able to adjust the maximum tilt of the clamp i added from the top, 2 screw over each clamp that act as end stops. I added latter tiny blot at the extremity of the screws, hold in place with thread locker. This way the loose clamp do not jam or slide to the side of the screws (the slot was maybe too large).
There is definitely an improvement to be made on the clamping system, maybe by making it accept the shorter square squeegee generally used for this kind of printing table.