While I think the Weber bottom will work as a charcoal basket, I think it will clog a lot easier as the as the ash settles under the grate and then also the ash that falls through th vents and onto the drum bottom. I would go without the SS pan under the basket and try to raise the kettle bottom so it sits a little higher than it already does. I would even consider drilling extra holes for more ventilation.
Try out your current set up and see how it works. You can always add more vents etc if it isn't reaching proper temps.
Good Luck DM.
UDS, PBC-Courtesy of Poobah, WSM, 22.5 Weber Kettles-A, K, DR Stamped, 18.5 Weber Bar-B-Kettle, Weber 18.5 Smokey Joe Platinum, Weber Genesis Silver B, Lil' Chief Smoker, Lodge Sportman Grill, Backyard Fire Pit, Dutch Ovens, Discada Set Up!
BobBrisket Jr.-MUDS, WSJ Silver
My BEST Compliment......"You're a good cooker, Dad!"--My Son :)
Miss You, Marty and Bull! RIP