Top weights do help. Restraints are awkward with marginal results. Check the temperature difference between the top and bottom layers. Typically there is a large difference and this will cause more warp. The only commercial use of weights I have seen was in a mill drying 2 x 4s of southern pine.
Weights do work, but their use is cumbersome in a commercial setting.
When stacked in our Nyle 250, there are two stacks, end to end, each eight feet long. My partner and I used to struggle getting 12 16x16x8 pier blocks on each pile. After all the work, we found we still had 3 or 4 boards on the top three or so layers warped. One time we tried just using the plywood without the blocks and we got the same number of warped boards without the weight. They had great results but their bundles were perfectly square with all even lengths.