I never had anything like it, but I know from experience that flak turrets are any bomber's nightmare.
Don't put missiles on it, waste of money and energy, they can't do much vs caps and BS missiles are so slow most snubs would be able to avoid them, get ~2 pulses to wear off big heavy caps shields, 1-2 (heavy)Mortars, 1-2 flaks to counter bombers and for the rest the fastest firing guns you can get, but not solarises, solarises won't do anything against caps and bombers can stay and snac you from a safe range, get primaries or secondaries instead.
RM battleship ? What will you use it/need it for ?
Incase it's fleet battles against (mostly) caps - Just do cerbs, prims, a mortar and flaks. Why ? Since the only effective controls for anti-cap duty on the turtle are ''move forward'' and ''shoot'' you will need good firepower and the unspecialised turrets get you a lot of it.