Problems with older drafting table

I recently got this older adjustable wooden drafting table. It's roughly a 3x5' 3/4" plywood top which makes it very heavy. Problem is that it has been slipping very often and I can see that the hardware is not very robust, specifically the tightening knobs that go through the base of the legs. It barely pushes against the long vertical piece and tightens just enough whereas not to put too much pressure on the narrow vertical pieces. Wondering if anyone had some tricks/hacks to solve this. As of right now, I put books underneath the vertical lengths to keep them from falling lower, but I like those books.

Similar style desk for reference