I've almost run out of containers for sprouting seedlings.
I use almost anything I can find, but my favourite is toilet rolls cut in half. With them, when the seedling starts to grow it's second leaves and start a root system, I transfer the whole thing into a bigger container and the roots grow easily into the new compost.
I start most seedlings off with sterilised compost so that it doesn't have to compete with the weeds in ours, then when it's big enough I transfer it into our goat shed compost which feeds the roots and brings the plants on ready for putting out.
I had to start again with a few of my seed tray inside the straw bales. I forgot that the chickens can't get in there and the snails and slugs ate their way through three trays full of nicely sprouting seeds in one night. These seedlings sprouted in two days, and they're safely out of the way of slugs on the back terrace. I cleaned out and lined the bale bed with plastic and I'll pop some seedlings in there tomorrow.