If you're looking for something mechanical to go between you and the amp you might consider the passive Classic Audio Effects roller pedal. It's a passive volume pedal that requires no power to function (but you can plug it in if you want the LED to light up) and it fits in a standard 1590B enclosure (regular old size pedal enclosure). I use one for electric bass but I think it might work for this application quite well. Instead of a fulcrum it uses a roller. Unique design and works great in these situations. I use mine to balance out my crazy high gain envelope filter.
You can find em on Reverb or used sites for around $80 USD.
You can find em on Reverb or used sites for around $80 USD.