I made this tiny cross bow from thingiverse
http://www.thingiverse.com/thing:1544I rescaled it in coreldraw to fit the 3.35mm thick plywood I had
I did this by drawing a rectangle 3.35mm high then rescaling the whole drawing until the slots were the same size as my rectangle
it worked though.
I don't have any foam darts though so it is a bit useless