They're the plastic boxes you get when you buy 'squashable' food items, i.e. cakes as above, soft fruit and veggies and salad items. They are perfect for seed sowing because they have a hinged lid which you can keep closed until the seedlings appear and are big enough to have it left open. Mine have a yellow reduced sticker on (naturally).
Fill about two thirds with compost, you will need to leave some growing space for the seedlings so they don't get crushed when the lid is closed. Water initially, then no need to water again until the seedlings are through because keeping the lid closed will stop it from drying out.
Add your seeds, here are some I dried out on a piece of kitchen paper last year, watering over this will start it dissolving before covering with more compost.
There you have it, free mini-greenhouses, pretty soon there will be a good crop of
blueberry muffins white strawberries in this pot, the other one contains tomatoes from seeds I saved from last year too.
It works, try it.