You could start by making the mouth frame from PVC. There are slide T's you can use for an easy way to make a hinge. I am working with Ctarpey to make his clown bend at the waist. You could use a scissor jack set up. The cylinder mounted in the middle like the jack screw. That way both parts of the jaw would come together like a real venus fly trap. I will draw up something for you if you like. Added note. I need to search to see if there is a double slide T. I started to draw it out and realized that to have both jaws pivot on the same plane a double slide T would be required.
Looks like no double T's. But I have an alternate idea. Cylinder in the mouth, mounted at the back of the throat. I will draw up for you. The drawing will show the mouth on one side fully open and on the other side in a broken line drawing the mouth closed. You will need a cylinder with a fairly long throw. Probably 6 inches at least. I figure you want the mouth around 3 ft in diameter. Like an Audrey.
I always imagined one of The Scariest sounds one could manufacture to be playing in a pitch black room would be..the obvious sound of a zipper being un-zipped!
You say, "Not Scary?"
How about if it continoiusly unzipped for a real long time, no pauses...and the imagination runs totally wild then!