If you put a wad of newspaper in the bottom, it will hold the soil in,
hopefully long enough for the roots to bind the soil.

Steve

I would agree with Steve, that a wad of newspaper will be enoughto keep the
compost in place until the roots bind it, the problem is likely to be the
other way round, that you must space the leeks out by several inches
depending on the variety, and how big you want them to grow, so you wont get
many leeks in a pot. You might be able to get a more suitable container if
you look around, perhaps a pile of old motor tyres? or the buckets that
chicken manure comes in??

Good luck with your leeks, I have memories of my geordie grandfather comming
home from his allotment with a leek over his shoulder!

John