Repair a rotted section of a porch column?

My porch columns are wood and has rotted in one column where the railings are fastened (about 4 feet up the post). The column is hollow and after chipping away the rotted wood, there is a hole about 3 inches wide by 8 inches long. How can I fill in the hole and then be able to reassemle the railing to this now rotted area?