Wow, thanks for the comment Wight. I think I'm going to like this Hobby and Forum...I've posted some pic's showing the Gun Mount detail of a 105 gun mantlet E8257 out of "Walk Around M4 Sherman"...It's a great source of reference and would make a great extra xmas present if you don't already have it.
As for my mod, it's made out of semi-circles of plasticard glued on to sides of the mount then the perifery filled in with white milliput to a concave section if possible... Use shaped wooden tools dipped in water to achieve a smooth finish,BBq skewers are ideal for this... The exact thickness of the plasticard is up to you. I tend to work by eye a lot... Just use your skill and judgement in the final choice...The gun and mantlet will almost certainly have to come off for you to work this area and whilst you are at it don't forget to fill the gap across the bottom of the gun mount with thin plasticard, cutting small slots in the card to allow the mantlet flanges to slide up and down freely...

I hope this helps.

Gigantohawk