The next batch...
These racks are not finished yet. I am working on three extension pieces per rack so that the height is adjustable from 1000mm to ~1210mm. The lower ballast board is now just a rough scrap wood job (still good enough to hold 200--300kg) and I will change that too. Will place my weight plates and KBs there. Due to the low center of gravity it is next to impossible that the racks will ever topple over. The two boards on top which secure the barbell are removable, and voila you have two 1000mm height jumping boxes.
That's a 21mm birch multiplex plate on top and there is no way anyone on any board squats enough to break this plates...
If there is interest I will write a short description how to build these racks including parts list and tools list.