Seemed to me, if I could just cover about 10"of the stem, the leaves would grow out the top of whatever I used, and they might end up being blanched, the way I hoped. I finally fixed on using 1litre milk or cream cartons, and I only had one to start with, so I threaded the leaves through it, and secured it to the earth with a landscaping staple.
Worked like a charm! Now I have another six to position tomorrow. The leeks won't be full grown until about November, and I should be able to use them through the winter. They're pretty hardy things. the most interesting thing so far is that the carton covered one I started with is much bigger now than any of the others! You can see this here.