i did one a while back i went off what this said boardlady.com/jppowerbox.htm but cut some corners because i didnt have that high density foam , i just cut the hole the exact same size as the new box and slid it in with a bit of filler . probably easiest and cheaper if you dont have all the materials and tools just to get ed (0402537715) to fix i though .
Thanks simon if I need that it's good to know, was after some help on changing a fin from powerbox to tuttle.