This is a very simple trick pumpkin that gets great results. It's cheap, easy, and very effective. The build is easy enough that even kids can do it and the trick is so reliable that even small children can scare the pants off adults. It works so well because everyone has seen one of these plastic pumpkins before and they're primed to reach right at the opening -- usually with candy!
I made one last year and took it around with my kids trick-or-treating. I'm an adult so many people answering the door looked at me suspiciously when I held out my "pumpkin". I just pretended to be collecting candy for my "other kid, sick at home". Even with suspicion raised, the trick worked great and I got invited into several parties. We 'test drove' the pumpkin at the annual Pumpkin Festival in Hartsburg, MO. Here's some of the reactions we got in broad daylight.
Not much to this really. You'll need a pumpkin, of course. You probably have one lying around but you can buy them for about a dollar at any place that sells Halloween supplies. The best one to use is the absolutely most common one available. Orange, plastic jack-o-lantern. The one everyone has seen a million times. You don't want your victim studying your pumpkin at all.
You'll need a pair of gloves that you can sacrifice. You're going to cut a big hole in one so don't use anything nice or expensive. I found a cheap 3-pack of cloth gloves for less than a dollar at one of the depot stores.
A couple of plastic grocery bags for stuffing material.
For tools, you'll need a utility knife a hot-glue gun, and a pair of scissors. A compass is handy for marking the cutout but you can make due without.