After doing some research, it seems as if the Energizer L91 batteries (sold as Ultimate Lithium) will work to power Hexy.
Can somebody please confirm? The spec sheet can be found here: http://data.energizer.com/PDFs/l91.pdf
A new pack of 4 has a total voltage of 7V, that should be fine? Seems as if the current rating is 3000mAh (judging by the Discharge Profile graph on the spec sheet, also: Max Discharge: 3.0 Amps Continuous).