I have the MightyBright Travel Flex as well as their Pocketflex and Flex 2. Of them all, the one I like to use best on my 600 is the TravelFlex. Plus, if you like reading in landscape mode, as I do, the shape of the TravelFlex's battery compartment actually makes holding the book easier. It will, essentially, just lay across your index finger needing almost no grip.
I have posted about it on this thread:
https://www.mobileread.com/forums/sho...t=57267&page=2