And, if the measurements of the objective bell and eyepiece fall in between two sizes, Butler Creek suggests going with the smaller of the two.
Butler Creek makes three types, and they all flip up, even though only one is called the Flip Open.
Flip Open, a harder plastic, not as flexible as the Multi-Flex.
Multi-Flex, has more "give" when installing, and is a bit grippier, my personal favorite of the three.
Blizzard, has transparent windows that fall out rather quickly, in my experience. Not a fan, YMMV.
There is also a more recent style, the Butler Creek Tactical, but I haven't had the chance to try them: