Lotus, over on HauntForum, built a simple pneumatic cobweb gun. I used his design to make one myself. I bought an 80-watt glue gun for $8.50 from Michaels using a 50% off coupon. I bought 1/4 inch copper tubing, a control valve and a connector to hook it up to my compressor. Total cost of those materials was about $11. So I've got less than $20 in the gun.
Like Lotus, I zip-tied the setup to the glue gun. That makes it easy to adjust the position of the air tube.
I'm going to have to play around with a bit more to learn the proper technique but the design works well. It does eat up glue sticks so I'll be searching for some cheap ones.