You can use M6 and M7 (screws and botls) to secure everything to the rails.
I've used only M6 and it's plenty solid.
The author remixed this model.
Changed screw hole pattern and shape a little to fit Gator Rackworks rails (https://www.amazon.com/dp/B071KSXY1H)
Center part hole pattern is different as it was easier that way since I was working with STL files