This might seem a bit painstaking, but I find it well worth the effort......
I use cat litter trays, as they are nice & deep, be sure to make holes in the bottom though - a hot skewer will do the job. Don't try to drill them - the plastic will crack! I use compost from grow bags, as it seems one of the cheaper ways to buy it.
Now here's the painstaking bit, I plant the seed, (mid-March), individually 1" apart 1/2 deep. That way they don't crowd each other out. I get nearly 100% germination in the cold shed.
One of these trays will produce 96 plants. Compost has to be kept moist. When a reasonable size they can be moved outside to grow on. I then transplant when they are the thickness of a pencil.
Another similar thing that I find useful are the polystyrene boxes that the greengrocers get Broccolli delivered in. They just chuck 'em away so they'll give you them for free.
Same idea,deep sided so you get good strong plants ready for planting out,and the polystyrene helps to keep the warmth in at night.
They come in all sizes 12"x18" to 2ftx2ft.and are handy for all sorts of other stuff as they have a lid on too.
But I'm not really into growing giants, just normal size will do for me
This really does seem a strange place to be discussing leeks though, thought this would be better off in the gardening forum, maybe it will get moved soon