I totally understand why you would want to move modules around. I also know what you mean about having a mini rack for focusing on a specific module without having it tied to your main studio. I have basically that same setup: a big rack in my studio, and a much smaller portable case I can carry around wherever if I want to concentrate on a specific module. I equipped it with some basic utilities and a few leftover modules that I bought when I first started out but have since retired from my main studio rack. It's about half full so there's plenty of room for testing out a new module. Also, it was a handy place to put modules before I completed building my full-size case.

I think my biggest hangup is the size of the cassette in the image, though the more I think about it it's a difficult balancing act. The larger the cassettes are the less advantage they have compared to individual powered cases. A large cassette forces you to swap many modules while you may only want to swap one or two, and it's actually a bit more complicated for traveling. A powered case can go anywhere that has electricity. Traveling with a cassette requires you to make sure your destination has a cassette-ready case waiting for you. You can circumvent that problem by bringing your own case, but at that point the cassettes are superfluous. The smaller the cassettes are the more interesting the module-swapping possibilities become. The design you created could easily be made narrower to scale it down. However, the smaller you make the cassettes then the more HP you give up in a large rack for the space taken up by the latches. And of course the cost goes up because you're having to buy more cassettes. I think determining the sweet spot in the cassette size is going to be all-important.

I totally agree that it makes sense to 3D print these before having anything machined, if it's one thing that 3D printers are good for it's this kind of prototyping. Also, now that I think about it, you could probably also make the cheeks of the frames out of bent sheet aluminum. Modern sheet-metal forming presses can insert hardware like threaded inserts, studs, standoffs, etc automatically so companies like Protocase, etc, could make those quite economically.

I am sure I could build a nice, workable, "flip" style case. The problem is that it would be a lot of work as there are a lot of little details to worry about. The latch on the flip portion needs to be well thought out because if it feels awkward to use or doesn't lock securely then it would ruin the ergonomics of the whole thing. Commercial-quality door latches seem like a good choice since they are meant to handle high duty cycles, but where exactly to locate them and how to control them? Where to put the lock handle and how to make it work? And there also needs to be some provision to balance the rack because if it ends up wanting to flip one way or another due to gravity that could make operation difficult. Perhaps operating the latch simultaneously applies a friction brake to ensure it doesn't move until you can put your hand on it? Maybe have some kind of rotary damper on the axle to limit its rotation? I'm sure I could address all those things, but it would be a complex, expensive project that is ultimately a gamble in the end. I could foresee it being perfectly functional but awkward to use, and I'm not sure I'm willing to roll the dice on that yet.