Two pieces of 1"(ish) pvc pipe with a sleeve of full-length 1/2" rubberized pipe insulation (not the cheap foam crap). Cut the two pieces to to about 6" longer than your vertical stab/rudder assembly total length.
Permanently bungee one end together. Removable bungee on the other.
Cheap and effective. I've owned two planes w/o gust locks and this worked well on both.
downside: likely not transportable in some planes. I could in both of mine...laid against one sidewall of the cabin, would fit to the side of the copilot's seat and under the back seat.
