UnO2 firmware gives some nice features for creating an inexpensive bass pedal:
you can define a variable which contains the transpose value - it can be incremented/decremented with 2 switches, and added to the note values of each note switch.
You can use a "wait" command to send fixed-length notes so that you don't need to keep the switches pressed
You can trigger as many NoteOn/NoteOff commands as you want with 1 switch, so you can play pre-programmed chords or note sequences
Instead of using 2 transpose buttons you can also create a bank for each scale, and fast-scroll through up to 200 banks