I've been saving the plastic tubs that "Bachelor Chicken" comes in from the supermarket: whole pre-roasted chickens. I usually don't buy tomatoes.
The black bottoms may be a little shallow, but the clear plastic dome is fairly tall and easily pops off.
I start most of my seeds in propagation trays or plastic 6-pack "inserts" or "cells", indoors under lights, but then I need to move them outside as soon as possible to make room for the next batch. And they'll need warmth, outdoors, for around 8 more weeks.
The plastic "humidity domes" large enough to fit over a whole 11"x21" tray are too expensive, so I'm planning on using plastic film draped over bent sections of garden fencing. That seems stiff enough to support plastic film despite wind, and I'll weigh down the film all the way around to keep slugs out.
When I vent it, I hope I can vent only the top, not the bottom, so slugs will have a hard time climbing the plastic to get at the seeedlings. And I'll sprinkle bait around the trays, under the plastic!
