Hi Kenny and thanks for your question. I understand why you need portrait mode.
However, the decision was made early-on to work only in landscape mode. At some point we might revisit this decision, and your need is quite compelling, but this won't happen anytime soon.
In the meantime, as a terrible workaround, most of the gestures in MIDI Designer will be relatively intuitive is the app is sideways. This is a crude hack, of couse, but might work for you in the meantime.
Thanks. Best!
Dan