Richard,
Williams Brothers makes these commercially...and Tower sells them. I have not seem them up close but it looks like your design is close....they use a rubber band instead of a spring. Maybe you can get some ideas from these links.
http://www2.towerhobbies.com/cgi-bin...?&I=LXKK27&P=7
http://www2.towerhobbies.com/cgi-bin...?&I=LXKK28&P=7
CraigA