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.