All you need to do for silly string is
1. Realize that you are the only one that really knows how accurate your pack is.
2. Take a small can of silly string:
a. wrap it in black Duct tape
b. cut the tip of the spray head off just below the hole that sprays sideways
c. take a small nut and place it over the spray tip
d. take the cap of the spray can and make a hole the size of the tip and mount it to the tip (This makes it easy to spray the string, plus you can reuse the same tip on other cans).
e. Strap the can onto the gun with more black Duct tape.
Note: You will have to shake the can a lot and it is best to only shoot at an angle upward.