This started out as a project to learn modeling in FreeCAD. I've attached the FreeCAD project file as well as the .stl models.
I'm not sure what I'll do with it, but it seems like it would work nicely in some kind of Halloween costume.
The main bracket works like a clip-on spray-paint trigger -- In fact, the bracket should fit other kinds of rattle cans as well.
The servo bracket was modeled to fit a hobby servo I had lying around. I've found that servos have a variety of different mounting screw patterns so you might have to modify this for whatever servo you're using.
A bent bit of paperclip works to connect the servo horn to the end of the trigger.
Here's a short clip of it firing: http://www.youtube.com/watch?v=fPpnoXX5sNc
I just learned that this design is used in the Icarus One spray paint drone by the artist/vandal KATSU.
Print out all three parts.
Snap the bracket into the collar on a can of silly-string.
Attach the trigger to the bracket with a 3mm screw. The bulge in the middle of the trigger should be pointed down and rest right on top of the spray can nozzle.
Tap the two holes in the trigger to 4mm.
Mount the servo to the bracket with the servoplate in between with two 4mm screws.
For additional rigidity, screw the other end of the servo to the plate with two more screws and nuts.
Control your servo with an arduino or other proto board.