I think one of the hardest parts of this is that the seat is going to be sitting on 15"s of bracket. Making a bracket is one thing but seeing how it is going to be visible when the door is open it will have to be visually appealing. Now you could build a bracket and cover it with stainless or polished aluminum. I myself would almost try to recreate a factory look. One option is if the factory shroud is tall enough you could build a bracket to fit inside of it. If not once you build a bracket take sheetmetal and form it around it. I would round the corners just like the one you have.
