I missed every opportunity to document this prop mushroom’s build, so you’ll have to just do the whole “theatre of the mind” business as I describe the process.
I produced the cap separately from the stalk.
For the mushroom cap I first cut a home trash bag into 3″ strips, then laid laid them inside large (18″ diameter) chip bowl as shown below:
By lining the bowl with strips of plastic rather than larger sheets, the plastic better matched the form of the bowl. I then mixed and poured some FlexFoam-iT!® V expanding foam from Smooth-On into the bowl. I used a GLOVED hand to evenly spread the foam.
I waited until the foam started expanding (which was my cue that the foam touching the plastic strips was tacky and becoming solid) then I picked up the bowl and tipped the bowl in a circular rolling motion to evenly distribute the foam.
While the polyurethane foam mush-cap cured I ran down to the wood shop and, using the jigsaw, cut the following “kit” for the stalk structure out of 1/4″ plywood.
As you can see, they assemble into a locking structure as shown below:
I then stuffed the void-quadrants with tissue paper, then wrapped with clear tape. I painted to match the pale yellow complexion of the foam.