I have them to add one extra grid onto the c&c as the nosey boys have started leaning over the one, so i have added a extra one for safety. I bought the ones from Wilkinson's they are quite small but i use about 5. One on top to clip to the other grid and then 4 straight down, i also tie the grid with a shoe lace and its stayed on fine!
I couldn't find any that were the right size when i needed some so i ended up using those stretchy wall plate holders ~ which was good at the time as i was a bit obsessive about the cat or dog getting at the pigglies and the plate holders gave a very strong tension to hold the doors shut. I don't need to use anything now as i've re-arranged the c&c's since the furry family has grown