@JangerClamping along the blue lines is to resist the force of the part, and the resistance through the hole should be greater...
I had intended (but I probably forgot to mention that it should be relieved along the red lines. I hope this is a little better explanation.
View attachment 23373
I like these improvements
@Janger . I'd also add a vertical slit at the bottom right.
What
@Dabbler s slits do is provide for some compression in the other columns. Those big steel columns need to do more than bend a bit - they need to compress. But the big steel blocks cannot be easily compressed with a small screw. The compression will be insignificant. The slits allow for some spatial compression in the gaps not in the steel. I'd make those slits quite long. In fact, size the remaining piece (uncut portion) about the same size as the screw. There is no point in bigger than that anyway. Lots of nuances in there about how fasteners work as well as the distribution of stress and strain in the part.
That said, I think two screws - one on each side - is a superior solution. This allows for 100% compression of the two side columns. As you visualize what the two screws do to the block, I think you can begin to see the issue with the one screw version. The two screws put compression into both side columns, (but still leave the top and bottom uncompressed). Of course the slits help solve that in the one screw version.
In my mind, I am visualizing a simplified finite element model of your parts. That is to say, break the components of the outside block into multiple blocks in your mind. When you do this, you can begin to see how each adjacent element influences the one next to it and so forth around the outside block. If you do this for your original design, you can watch (in your mind) the top bar bending (somewhat like a cantelever) down on the right. But there are only very low forces to pull it to the right as the screw is tightened. The only forces doing that are the restraining forces in the left column that are trying to stop the top bar from bending. They are somewhat like the anchor for the cantelever. But this effect gets dramatically reduced for each leg. In addition to providing space for compression, the slits help turn the hinge points into quasi unrestrained fulcrums for the other three joints.
The two screw version provides raw vertical clamping but no side clamping.
All this said, unless the complete assembly is different than what I think it is, I still don't see why clamping is required at all. I would just make it a vertical slip fit and let the backlash look after itself as the outside jaws are tightened. Think of this a bit like a part put between two (or three) jaws. As the Jaws tighten, the first ones contact the part, and then the part slides until it hits the second, and then the second and the parts slide until they hit the third, and then when no more sliding is possible, compression clamping begins.