I've just built a set of side pipes for mine. I've used a slip joint system but the clamp is part of the pipe where is slips over the flange and is invisible. With a bit of high temp RTV it should seal well. I've built mufflers this way for my bikes in the past and had no problems. I've also put cat converters in them as they are required for emissions down here. They are part of the baffle structure and are held in with grub screws so I can remove them and repack them. The baffle is a 2-1/2" perforated tube that's welded onto the back of the cat (I haven't got a pic of this yet). The baffle is wrapped with stainless steel swarf and fiberglass and is easily repackable or I can unbolt the baffle and change it for a different design. There was a bit of machining involved in making the flanges but I think they turned out nice.
Hope this gives you some ideas. There's more pics in my gallery.
Cheers