aftermarket Magnaflow resonator (2.5 in. OD shown here) comes in various lengths and diameters. Stainless steel and aluminized steel. Stainless resonator though is shorter. This is a straight-through design resonator. A deign which doesn't add backpressure or impede exhaust gas flow speed (i.e. a good thing from performance point of view).
here it is on the Magnaflow 2.5 in. catback vs. the stock 2.25 in. OD ITR catback. You can have a local muffler/exhaust shop weld it onto your aftermarket catback (using the correct diameters of course).