How do you organize/mark your Eneloops?

Hi,

I just got 8 more Eneloops for my Flex TT5 and need a way to mark them in pairs. I've used tape before but then had problems with the batteries sliding into my Nikon flashes. Also used a Sharpie but usage made ink fade.

How do you organize/mark your Eneloops? I have about 60 now. Do you need to keep sets in order or can you mix batteries? I am using a 2 Maha MH-C800S to charge all these batteries. Just want to make sure I get the most out of them.