I’m shooting from the hip here, but GoPro Studio is a free software with a fisheye correction feature. Of course, it’s designed for the GoPro camera and may not be as flexible as you need, but it’s free.
only tested the 64 bit one but i included a 32 bit as well. i made 46 the default which seems to work best for full hd comps. 50 is no distortion. for other scenarios remember that it matters if you place it before or after Transform.