I went with this one, because I wanted a particular height that would be useful when I'm sitting. But if you want more space to mount things, or if you're going to be standing, the taller ones are really not much more expensive.

I bought a 2ft x 2ft piece of 1/4" plywood and cut it down to 19x20 for the top (and painted it). That makes the top a workable surface.

I also went with a pair of these shelves, for effects pedals. They also fit a single SQ-64, or Akai MPK mini, or two Volca units per shelf, so they're really nice depending on how you work.

And I organize my manuals, screws, cables, and accessories in this drawer.

I've also found some various clamp-on mic stand accessories that have helped to further make this a really versatile rolling studio.

Of course, lots of zip ties in various sizes, in order to strap down pedals and power and USB hubs, and to organize cables and keep it all tidy. and prevent things tangling under the wheels as I move it around.