Thanks for the compliment Freddy. We use a rolled 16 or 14 gauge galvanized ring 2" wide with tabs spot welded on it for welding to the top. Simply place the ring in the location you want to vent from and cut out the circle using a plasma cutter or other means. The price for the tops will be set by your local dealer, if there is no one close enough to warrant the trip PM Flyer or I and we will get you set up, probably considering your vast fabrication abilities a flanged ring of your desired size will be the easiest and least expensive option.
Enjoy your new find!