We have a wacky theme for this year. We're building giant bobble heads of classic movie monsters. The heads are foam, weigh between 5-10lbs, and are about 30 in. tall. I plan to use a pneumatic piston to nudge them from underneath to make them bobble on command, but I'm having trouble deciding the best spring to use. Obviously they need to be fairly big and robust, yet capable of at least a little wobble. I tried the tension springs that are used to mount above garage doors but they're far too stiff. Replacement springs for trampolines are too short and screen door springs are too skinny.