regulator help (for bike lights)

I'm curently getting 14.6v with one bulb with batteries which have been fully charged then left for a day. It drops to 13.9 with two bulbs though and would like to get a regulated 15volts to the bulbs. the power supply is made up of 24AA cells 2parrallel by 12 series but I could change it to 24 in seriers if that would be better for adding a regulator to the circuit. I don't really expect to run both bulbs at the same time but it would be nice for to have full brightness for 2-2.5hours (I'm running 20w IRC MR16 Bulbs so would need to draw about 4amps on full power).

Unless regulators are really simple to make I'd rather buy one from someone on CPF or from a shop (thuogh I'd need it posting to the UK)
I'm guessing some of the amazing mag mods will use something similar though size isn't such an issue to me as I should have a fair amoutn of space for it in the battery holder


Thanls
Andy

I think andrewwynn makes regulators for incans (the "hotdriver"?). Find one of his posts -- I think there are links in his sig line...