Good for you! Though I've got some sympathy for the guy who did: it's all too easy to design something to work under particular circumstances, even if it's really meant to be more versatile, and then someone tries to run it at the end of its range, and it's "Gotcha" time.Well I did not design the circuit or the board.
Could anyone show me a schematic of a stupid-simple switching regulator circuit?
The LT1076 circuit that Ifixit posted is pretty typical of switching converters, and my guess is that little $25 module that you found has something similar inside it, with the two resistors replaced by a pot.
http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1042,C1033,P1007
You could build your own equivalent of that, and if you're making 500 of them, the cost shouldn't be too painful. As I said before, you could customize its mounting to fit your existing layout.