Interesting suggestions on here.
I currently use an Akai lpk25, that’s about as compact as you can get… but I’m not sure I’d recommend it for the MM.
Due to it’s tiny size, it does not have a dedicated midi port, only a computer usb, which means that I depend on the computer and audio interface to run it with the MM. You cannot plug it directly into the MM.
Maybe there’s a converter/adapter that will remedy this, I haven’t looked into it.