?? I don't really understand what you are asking, Doug. Leeks are grown very much like spring onions, they just get bigger. Is the collar to blanch them, as in show leeks? I earth ours up as they grow but I suppose that you could put a collar round them. I am not sure what you would use, though. Paper would make an easy collar but would get wet and go soggy. Plastic would not rot like the paper but could cause the leeks to soft underneath the collar.

I haven't done it with leeks but for other plants needing a protective collar I use loo roll inners. They last a reasonable length of time ...

I make a deepish hole with a dibber (eg a broken spade handle), put the young leek in the hole and water it in. This almost does away with the need for a lot of earthing up. The soil gradually falls into the hole as the leek grows.
Isn't this a lot easier than putting collars round them?

4 " diameter plastic drainpipe, 12" long, secured with 3 x 3 foot canes.
As the leek grows, add another pipe should you wish.

Gosh Scrumpy, your leeks must be impressive!

That's what i did years ago. Haven't grown them like that for a long time. To get decent leeks nowadays you need polytunnels,etc. Too much hassle.
Good old fashioned planting deep now.
Corrugated cardboard can also be used.

Luckily I'm content with little ones!