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.