Why not just treat the entire IV as a big integer with that many bits? With AES-128 the IV is also 128 bits, which is effectively infinite. It's roughly the same number as the number of sand grains in the visible universe.
It's simple enough to implement as well, it's just two 64-bit additions with a check for wraparound on one of them.
It's simple enough to implement as well, it's just two 64-bit additions with a check for wraparound on one of them.