One morning I saw a box from some Jolly Jammer biscuits, and it occurred to me that each side of the box was about the same width as the tabs, that the cardboard has a varnish finish, and they might just help the tabs to slide. There was only enough for four tabs in that biscuit box, so my mind started to think laterally.
I looked at an empty cereal box. Yep, I thought that could do.
Then, after exhausting that material, I looked at an empty milk carton and I thought that might do.
Then I looked a a plastic milk bottle and thought that might do.
After trying all of these, I am inclined to think that the plastic milk bottle is probably the best option. One nice thing about it is that it is ribbed, and so the contact between it and the curtain rail is reduced more than the cardboard.
All options work, but my vote is on the plastic milk bottle.
I first simply stapled the material inside the tabs, but when I am resolved about what is the best option I'll replace the staples that are a bit reflective, but otherwise not really visible, with thread. But I am happy with the experiment! I thought I;d share it with others as they might also have the same problem that I had with the tabs not moving easily on the curtain rails.
Milk carton, inner surface inwards and outer surface inwards.
Cereal box
My vote is on the last picture - the plastic milk bottle or milk jug.