Leeks

I think you'd want them to be in something deeper, unless you plan to earth them up as they grow. If you grow them first in a deep pot them you can wait until they are quite large before moving them to their final position, planting them deep and blanching a lot of stem without effort. If you grow them in the seed tray alone they'll be smaller when you move them and there won't be as much stem to put below ground. You'll need to earth them up to blanch the stems as they grow.

We start ours off in black flower buckets, the type the supermarkets sell on after selling the flowers. You usually get ten for 99p

We start ours off in the deep tubs that grapes and other fruit come in from the supermarket. In the past we've bought leeks in seed trays before and earthed them up as suggested above so they should be OK.