Paul, you have lots of options. The standard fit for the Sherman V including Fireflies was the large, weirdly-angled box fitted to the rear plate. This is very commonly seen, and occasionally it was mounted using only the top two mounts and stood well above the engine deck. Presumably this was done to clear the deep wading trunk. Similarly, it could be mounted on the front trans housing for that reason. There was also another smaller rectangular box manufactured for CW Shermans that attached to the rear of the turret. This too was seen on all types of Fireflies strapped to the rear of the radio box.
Other oddities include a Firefly Ic seen with the large Vc rear box attached to the radio box; various ammunition bins on the sides or top of the radio box, and ammo bins on the front fenders. Also, units in Italy sometimes attached the Crusader-style stowage box on the rear or top of the radio box. All of these things can be seen in Haward's Firefly book if you can lay your hands on it. HTH,
Greg