I was thinking more of a foam and hardboard prop, a manhole cover made of foam with a hardboard bottom the mechanism can bang against, and a large piece of foam done up to look like a square patch of cement or pavement the manhole cover sets into. A MIAB mechanism can be made pretty compact with a short stroke, so the hole doesn't need to be that large. if the piston arm is made of metal and some bits of chain are added to manufacture the metal on metal sounds, the overall prop won't be too large or need to big a hole. A pop up on the other hand, you're right, it would be a nightmare to set one of those up under a manhole.The only problem with a popup from a manhole cover is that it would be way easier for most people to build a raised sidewalk area with street drain at ground level than dig a hole the size for a manhole cover with prop down below. If you were only adding a pvc pipe with fog coming from it, it wouldn't require much digging to hide the pipe. More likely approach would be the street drain where the street drain could have fog coming out of it or even have an arm or tentacle reach out from it for a surprise.