I also had this issue and I found a great solution. For $100, buy the Wholly Roller from Boomerang. Use this as your expression pedal. I have attached a photo of the Looper block configuration that works for me as well as a photo of my board. It works perfectly. If you have any questions, let me know.
I just prefer the wheel to adjust the level, I got used to it when I had a real Boomerang looper pre Fractal. I can roll it in the perfect volume very quickly.