If you stick to 4 space, Gator makes a cool gig bag rack case that is pretty cool for this application. Just unzip the front cover, fold underneath the rack case, and set on top of your cab. Standard rack cases may or may not work well in terms of safety/security. When I put an SKB rack on top of a cab, I took no-slip rubber sheets that I bought at home depot, cut them into strips, and glued them to the bottom of the rack case, so it had a much better grip on my cabinet. These days, I do it safely... rack case NEXT to my cabinet, head on top of cabinet.
I have taken Mesa Racks and simply added another set of feet on the bottom, in the middle of the rack, to sit on the cab better. If the front is flush with the cab and there is nothing weighing the over hang in the back down it is extremely secure.
Agreed - I must say that when I was running a Fractal rig I had a great OSP rack that I bought from an eBay vender. It was really well made and fairly priced.