Matt i have tried some buck convertors very simular and gave of alot of noise , i since then have bought a 30A bridge rectorfier the type you get in a generator and mounted it on a heat sink and i can get a 4s lipo down to a more useable 13.5v .
But its all weight and bulk , so i have built myself a pack from 18650 lifep04 cells its a 4s but 16 cells so 4 in series then 4 banks in paralell which gives me 13.2v . im not 100% sure of the ah but before i removed the wraps to re wrap the cells i saw 1200mah or 1600mah but was a while ago when removed the origional sleeves .Tey was from a 12ah lifepo4 battery which gave up but after testing was the BMS and some cells was at fault .
Will post a few pictures shortly
73 Matt 2E0FGX