I have seen the stock flex connected to a simple piece of 3" PVC or aluminum tubing that is painted look real nice.
Preliminary fit of mine, which uses the basic Lowes style hardware. Working on the intercooled outer sleeve design similar to others, which will be red. The only difference from the GP setups is that it makes the 4" overpipe fit difficult because I am not using the 22.5° elbow like the Grand Prix fitment, bonnevilles need 45° to hit the fender hole.
I went from stock box with KN panel to gutted airbox with 7" KN cone, then HAI 9" KN cone in the engine bay for the winter, to this with 9" KN cone in the fenderwell, and it finally made a night and day change.