Don't get me started on this one... For me, using household scraps is part and parcel of "livin' the dream" that I've had since 1957. I have been known to find some random bit that has a provocative shape, and keep it around for years until it finds its place.
Here are a few:
I was walking down the street one day and picked up a strange, shiny plastic silver ball. It eventually ended up as a lawn ornament:
This brown "tank" enclosed with other Important Equipment was a medicine dispenser of some kind that I saved for maybe 20 years because it looked so "tank like":
Speaking of tanks, the one in the rear with the big pipe coming out of it was the cap from a big jug of laundry detergent. (The other tanks are PVC plumbing fittings).
Interesting shapes don'e even have to look like anything in particular--we see stuff in RL all the time that we can't identify:
And, then there is my infamous "vegetable basket window muntins":
And, of course, if all else fails, there is always the scrap yard:
