The binder clips are really only good if the rabbit won't push on the door to get out.
Small (10") bungee cords can be used, but some rabbits will chew them. I weave them through the NIC grids and leave enough for 2-3 squares to close the door.
Dog leash type clips would probably be the most secure. They can't be chewed off and shouldn't come undone. Make sure you get them large enough to be able to fit over the edges of the door. You can also try getting 2 smaller ones and zip tieing them together, this would not be as secure, but can work in a pinch.
Caribeaner clips are also good. They won't keep the door closed tightly, but will keep it from opening. You can get them at dollar stores for a couple dollars and they usually come in 2 or 3 packs.
I use a mix of all these, what I use just depends on the cage and the rabbit.