Thinking about it, I cannot remember seeing a single aluminium extrusion-based machine using diagonal bracing under the bed (although that might be a faulty memory!). And some of those machines have certainly been capable of machining aluminium. Additionally, the biggest "shaking" loads are involved in accelerating and decelerating the gantry, followed by accelerating and decelerating the Z assembly along the gantry. That is much less than the first, obviously - the mass of the gantry is the biggest load. I've reduced the effect of the second load by raising the side rails so that the Z assembly is not as high above the X rails - moving mass is more in line with the support points - although that means that the rails need good support. I think I've achieved that. I instinctively feel concern about the designs that use a single tall plate each end of the gantry, which I can imagine bending under dynamic loads. Clearly I'm wrong there as there are plenty of successful machines that are built like that although they need pretty heavy plates to take the bending loads. But engineering is all swings and roundabouts and there are always trade-offs to be made. My gantry design is more fiddly to make, for example, and probably needs a vertical mill as a minimum (which I have available, hence the design).

I do have a single diagonal corner to corner across the end (shorter) sides of the main frame. I have two more-or-less diagonals across each long side, but these are arranged to take the load of the intermediate X rail tube supports down to the bottom of the legs and do not run corner-to-corner. That does leave space to get the control cabinet in (although to be honest I had forgotten that when I built the frame, and I was lucky that the cabinet fitted the space available with a few millimetres to spare). Occasionally the gods smile on us.

I don't have a support leg in the middle of the bed, and I happily crawled over the bed structure when I was building the machine. My bed is all 50x50x3; if you used your 100x50x4 as cross-rails and braced it with intermediate 50x50 segments between them, I'm sure that would be stiff enough.

I would advise having a suitable device for cutting the box section to length accurately and squarely; I used an angle grinder in a small pivoting stand and struggled to achieve accuracy. This was a bad decision on my part, born out of lack of experience. One of those cut-off saws, maybe, with a decent size blade? Personally I would go for a metal-cutting bandsaw but that's because I have other work I do that would make use of it.