Thanks guys! I don't want to break a seat rail.
I stopped by the bicycle co op yesterday and found a narrow-ish standard two piece seat clamp for a dollar. It's slightly wide to fit behind my seat clamp on the rails. I'd need to grind it down a little to get it to fit. I'd rather clamp to the seatpost, but I don't want to damage any seat posts.(I also have a bike with a dropper post)
Steel seat rail clamp