This is a trick learned from CRASS stencilers.
paper grocery bag
side of half rack box sized stencil
maybe bluetack or something else to affix the stencil to the wall temporarily. i just use tape.
Step 1: Cut the bottom out of a paper bag...
make the hole slightly smaller than the stencil so as to have something to tape on, and give it a little strength.
Step 2: Tape the stencil to the INSIDE of the bag
if you leave alot of space around your design you'll have enough support to keep your cans in the bag itself. best to use the small model sized cans so you can move your hand around in there.
Step 3: That's it! now you've got a normal looking paper bag
with the voice of errant youth in the bottom. put the bluetack/tape on the bottom, press it up against a wall, and spray! ta-friggin'-da...