If you’re not having much luck recording MIDI parts with a keyboard or pad controller, why not consider Zeppelin Design Labs' Altura Theremin MIDI Controller? This has just been given a MkII upgrade, which brings an arpeggiator/step sequencer into the mix.
As you might have guessed, the device is played in the same way as you would any other Theremin - ie, by moving your hand - with the arp enabling you to program sequences of up to eight steps, including rests and sustains.
Sequences can be looped and played up and down the scale, and you can lock to a selected key to ensure that bum notes aren’t a problem.
Find out more on the Zeppelin Design Labs website. The Altura MkII is available in various formats - from a DIY kit to a ready-to-use device - with prices ranging from $20 to $169.