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FWIW, I made my bed from 2 layers of 18mm birch ply about 20cm wide, with a 2x4 screwed to the inside to support the slats. Each of the 5 legs are about 60mm square, there's another 2x4 running down the middle of the slats (for the 5th leg) and I put a small triangle brace in each corner to prevent racking or twisting. It's all held together with 3mm wood screws so that it can be easily disassembled. 10years later it's still rock solid.
Our current one (from Made I think and probably bought in winter 2018) had a leg in the middle underneath a metal supporting beam. The bed was dragged by my father in law when he, uninvited, decided to look at a plug socket behind it. The middle leg snapped. He didn't mention it. When I sat on it a bit too heavily the bed frame cracked. In fairness I weigh a lot more than I did at 23 but still. My ambition is a rock fucking solid bed that I can throw myself onto from time to time - so think a diagonal and downward force. And importantly one that doesn't squeak at the slightest tremor and one I can mount without having to spread my weight like I'm crossing quicksand for fear of it collapsing.