I saw this solution for flex joint at wastegate. real cheap and simple fabrication. you can clamp the stuff in place or weld it up. I would weld it up at the downpipe and weld the wategate side to the flange that came with your wastegate. be sure you clamp the wastegate flange flat and securely at both sides before you weld the coupler or you will warp the flange and need to get it machined flat. ask me how i know.
http://www.034motorsport.com/product_info.php?products_id=252
http://www.034motorsport.com/product_info.php?products_id=252