What you will need:

1 12" tube 18-24" high
1 2 x 4 sheet of 3/4" plywood
1 12" pneumatic cylinder and all hardware
Foam head, Ghost Head or a mask
1 4' 3/8" metal rod
14" piece of 1" metal bar
6" piece of 1" metal bar
1 4" and 1 1.5" bolt w/ washers and nuts
Wood stain
A roll of chicken wire
Fiberglass cloth, resin, & brown pigment
Paint
12-14" black light w/ fixture

The first thing you need to do is draw and cut out a 16" circle with another 11.75" circle cut out of the inside (if you're not using 12" form tube, then these dimensions don't apply). Now draw a 18"circle with a 14"circle on the inside. Before cutting these out freehand draw an outline of a tree stump around the 18" circle. This will be the template that you staple the chicken wire too. Now drill a small hole on the outside of the 14" circle large enough to fit your jigsaw through and cutout the inner circle and stump outline. Once you've finished cutting out your circles and patterns, take the first 16" circle and screw it onto the stump pattern. If you drew the 14" circle inside the 18" you have a guideline to keep everything centered (see fig).

Now take your form tube and cut it to 22" or your desired height. Take the stump pattern and screw it onto your tube from the inside. Apply a quick coat of white paint to the inside of the tube and when that's dry apply a couple of coats of fluorescent yellow paint. This will create a more spectacular glow under the blacklight.
You are now ready to mount the pneumatic cylinder to the tube and the lid. Before you mount either of these, you will first need to cut a 6" piece of 1" straight metal bar and drill a hole a 1/2" in from the edge. The hole will need to be the same size as the stroke rod on your cylinder. Now drill 2 other holes in the bar, one in the center and the other at the opposite end from the stroke hole. Now mount the cylinder to an 18" piece of 2 x 3", then put the lid of the stump into its resting position. Make sure that the cylinder is perfectly level, of the lid will open at an angle. Now put a drop of paint on the tip of the cylinders threaded end and then slide the cylinder and 2 x 3 up the inside of the tube where you consider to be the back. The paint mark will be where you need to drill a hole half way through the wood to accept the cylinders mounting nut.

Once you have drilled the hole, flip the entire stump onto its lid and mount the metal bar to the cylinder (you will need to drill a mounting hole for the cylinder as well as a couple additional holes to mount to the lid). Now securely mount the cylinder to the wall of the tube with washers placed on the outside of the tube. This will keep the screw heads from tearing through the cardboard tube and ruining your prop. Once the cylinder is mounted, screw the metal bar to the lid. Once that is secure, you will need to insert guide rails to make sure the lid closes in its proper position. To do so, take your 3/8" rod and cut it into 2 16" lengths. Now with the stump standing up and the lid closed, drill two 3/8" holes, one on each side of the cylinder (approx 4-6" apart). Now open the lid and widen the inside guide holes (not the lid holes) to 7/16". This will allow for a smooth lift and return. Now slide the rods through the lid and into the guide holes. When flush with the top, lift the lid up an inch or two and drill a hole through the side of the lid into and through the rod. Now drive a nail into each hole and secure it into the inner lid.

You're now ready to start applying the chicken wire. Apply the chicken wire to mimic how a tree stump looks.