Brassica collars

I have just planted broccoli on the plot under a netting cage.  To make extra sure that there are no problems with cabbage root fly, I have also put little collars around each plant as recommended by all the books.  I made the collars out of a hanging basket liner.  The question is, the collars are quite rigid (as would be the roofing felt or pond liner recommended by the experts) and I just cut a slit in them to fit around the stem of each plant.  As the stems get bigger, will the collar damage them by cutting into them?  Do I have to constantly increase the size of the slit in the collar?   :?