I used a power bank I had on hand- just a garden variety power bank you can get on Amazon or anyplace. It is similar to
this . The main thing is that it is low profile to mount under the 529 on top of the board.
I dont know why you need a USB adapter. Standard USB cable goes from the battery pack to the MI 529. You only need a cable adapter if your pedal is not center negative, which is how the 529 is wired.
For a board I looked at the nano and others but wound up with
something designed for wah/volume/expression pedal platform. The holes allowed me to use cable ties to anchor the pedals. Its very compact with clean routing. Too bad I no longer see the link to upload images!