The little plastic ring that hold the Tenebraex covers on the NF oculars constantly break, every one of mine did.
I'd recommend a NF bikini cover zip tied to the tube. It works very well and you can pop them off just as quickly as you can open a flip cap.
The Armageddon Gear Trojan cover also works well for keeping the lenses clean and doesn't create a sealed environment so fogging/moisture isn't an issue like it can be with caps. It also keeps dust out of your action while being able to show open bolt condition.