I use a very simple tiller lock on my P-15 that has served me well.  It consists of a long, heavy bungee cord fastened from one stern cleat to the other, and a jam cleat mounted on the underside of the tiller and about 8" forward of the bungee.  To use, simply pull the bungee up onto the jam cleat while on course.  I've found it easy to micro adjust, and have cruised hands off for upwards of 10 minutes this way.  Considering the greater directional stability of the M-boats, I think it would work even better for you, and the price is right!

Steve Tyree, P-15 #2098 "Amy Ann"