Some of the Sunn amps like the Sonic I-40 had the smaller cabinet that they came with like the one you show there. It is basically half of a 200S cabinet.
I built myself something similar....I made the width when the cabinet was upright the same width as a 200S head. The height when the cabinet is upright is the same width as a 2000S cabinet, so that when I flip the cabinet I made on its side then a 2000S head will fit on it. The port is along the bottom and is the same width as a 200S cabinet, but the height of the port is half of a 200S cabinet port to keep the same bottom end frequency response. The bottom of the cabinet is done just like an Ampeg SVT cabinet so the back part of the cabinet has an angle cut into it where wheels mount and the cabinet can be rolled around. There are handles on the sides that are the same as a Marshall 4x12 cabinet. It has a JBL E140 in it, and then later I added a couple 6 inch tweeters too that I had Conrad voice a crossover for. I also had to greatly increase the internal support as the cabinet was too loose and would move itself backwards across a garage floor in a loud amp head was on top of it. It was pretty funny actually, but after the internal bracing it is rock solid now and stays where you put it with no cabinet rattles. With the different speaker in it, of course the cabinet sounds different than a 200S cabinet, but it actually sounds really good and I like to record with it. I can provide pics if you want though I don't have internal pics. The cabinet is front loaded btw, and it is filled with lots of damping material too.