Introduction: Build a Giant Tentacle Monster

About: Web developer by day. Gamer by night. Halloween fanatic and DIYer, all the time! My projects tend to combine pop culture, technology, and craftsmanship to produce something that's fun, unique, and more than l…
This Halloween I wanted to build a huge outdoor decoration to compliment our "End of the World" party theme. We have a sewer manhole cover in the corner of our yard, so the idea came to mind to build a large tentacled creature appearing to crawl out of it. When completed it was about 6 feet tall and around 30 feet across.

This is a relatively easy build, makes a big impression, and should be versatile enough for lots of Halloween displays.

Rough list of materials:
  • 1/2" plywood - 2' x 2'
  • 1/2" foam board - about half a sheet (4'x 4')
  • 1x2" lumber - about 2' worth
  • 8' steel studs - 3
  • Vinyl trim (like flashing) - about 6' x 3.5" wide
  • 8" diameter HVAC flex duct - 25'
  • 6" diameter flex duct - 25'
  • foam pipe insulation - various diameters, 3 pieces
  • screws, latex caulk, liquid nails, and paint
  • fog machine and spot lights for extra wow

Step 1: The Base

The first step is making a base for the monster. In my case I built something that would fit exactly on top of the existing sewer cover. If you're just setting something in your yard, the construction of the base doesn't have to be as particular. 

Note: If you are fitting something over a real sewer, realize you run, a likely very small, risk of having your utility company take exception to it and having to move it.

I started the base by cutting a 2' circle from the 1/2" plywood (a jigsaw is a good tool for the job).

Next I cut a 1x2 board into eight 2" lengths. Then I screwed these around the edge of plywood (see photo). These pieces act as standoffs for the plywood to sit flat since the real manhole cover (which will be left in place underneath) is slightly domed. Then everything was painted flat black.

Step 2: Adding the Armature

In model making and sculpting an armature is the skeleton, or framework, used for support. I choose to use steel studs (available from most home improvement stores) to build my armature because they are light, sturdy, and surprisingly easy to work with. 

Safety tip: Make sure to wear gloves during this step; it's all to easy to stab or slice your finger open.

Using a pair of tin snips cut, or slightly notch, the sides of the stud, then bend them to shape. I started by screwing down a section of a stud about a foot from the end to the top of the the plywood. Then I folded the ends together, overlapping the sides, and fastened with sheet metal screws. Working my up I notched, bent, and fastened the stud every so often to form a curved shape. I made three "arms" this way jutting out from the plywood at different angles and heights.

Also shown in this step is building up the outside of the fake sewer opening with a length of vinyl trim (this was some sort of flashing also found at a home improvement store). This particular piece of vinyl has two ridges running down its length which made it easier to center over the short 1x2 boards. I painted both sides of the vinyl, wrapped it around, cut it to length, then screwed it into place (through the vinyl into the 1x2s).

The final picture shows a test placement over the top of our manhole. The boards rest solidly on sewer and the vinyl extends around the outside, down to the ground.

Step 3: Fleshing It Out

With the base and the armature done, it's time to make the tentacles! This was easily accomplished with flexible HVAC ducting. I bought 25 feet each of two different diameters (6 and 8 inches). The different diameters are easy to transition between (just join the sections with duct tape) and it'll give you a nice tapered effect. You could even add 10" and 4" varieties to mix it up further. 

The ducting slid right over the steel studs and was secured to the plywood side with a couple of screws. For the most part I was careful to make sure the ducts came right down to the plywood, but with my rear-most tentacle I left a good gap (an inch or two) between the plywood and the ducting. This was well hidden by the front tentacles, but would allow fog to come from the back tentacle and float out over the sewer opening like steam (more about this later).

I already had some decorative fencing up (built in a somewhat broken state in spots), so this gave me something to attach the other ends to. You can really get creative here, making tentacles wrap and spiral around something, as if the monster is pulling itself out. Or pulling something in.

The ends of the tentacles don't need a lot of special attention. Just gather the end closed, secure with duct tape, then push that taped wad inside the tube. 

Time to break out the spray paint! I used two colors to give contrast between the top and bottom of the tentacles. The paint easily covers the foil ducts, but can flake off after it dries, so avoid re-positioning the arms once painted.

Step 4: Adding the Sewer Cover

To help sell the visual of something crawling out of a sewer you need the manhole cover somewhere (remember the real one is safely in place underneath).

I fabricated the manhole cover from 1/2" foam paneling. I started with a 2 foot diameter circle and added pieces to that. The series of photos show the progression. After all the pieces were glued in place with liquid nails, I tapered down the outside using a palm sander to give it a slightly domed shape like our real cover.

Then I smeared latex caulk around every seam to make it look more like a single metal casting.

Foam letters were cut and glued down (spell whatever you like), then everything was painted. First I brushed on flat black latex over everything (remember most spray paints will melt foam!). Then I sprayed a couple light layers of metallic, hammered-style paint for texture. A little lighter metallic was brushed on the letters to make them show a little better at night.

Step 5: Finishing Touches

What good would tentacles be without some sort of suckers?

I bought 3 different sizes of foam pipe insulation (1/2", 3/4", 1"), cut them into pieces 1 to 2 inches long, then painted all those pieces bright yellow.

With all the bits sorted by size, I began gluing these to the underside of the tentacles with liquid nails. Larger diameter pieces went toward the sewer, and smaller toward the ends. I think I used around 250-300 pieces. It's impossible to get this wrong and the extra detail is impressive!

The last steps involved mounting the manhole cover in the grasp of one of the tentacles, setting up a couple spotlights, and the addition of a fog machine. A small fog machine sits in the shorter, lower tentacle and faces toward the sewer opening. A timer dispenses occasional bursts of fog which emanates from the base of the tentacles like sewer steam, or maybe smoke from the ruckus below. Since the tentacle itself is a duct it's perfectly suited to carrying fog from point A to B.

Safety Note: Before placing a fog machine in a confined area, review how hot its surfaces become in use, and situate it so nothing is making contact with a hot surface.
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