I'm very pleased with my Mr. Whoopee auto retractor, but there's always room for improvement.
I've been looking at the design with an eye toward moving or removing some of the stuff that protrudes on the headstock side. My first thought is that perhaps a compression spring could be placed between the plunger and toolholder body to eliminate the spring on the side. Then there's the lever that pops out toward the headstock when tripped. A different arrangement would be needed, and the only one I've really thought of would be less convenient to reset. The unit works well as-is, so it's mostly been idle thought at this point.
GsT