I share your grief, mate. I assume you also have the related P-code showing on your dash and you have actually confirmed by visual inspection that there is nothing else causing the rattle. It is possible to buy CATs that will fit exactly like the OEM part or 'universal-fit' CATs that can be made to fit with additional pieces. However, many aftermarket CATs, even if they fit perfectly, may still trigger the P-code and / or not pass emissions inspection because they do not perform as the OEM parts. If you can confirm those CATs have worked for other Infiniti owners without triggering the codee, I'd say they are worth a try.
If you're wondering about actual OEM parts, you can buy OEM CATs direct and save the exorbitant mark-up from dealer supplied parts. Here is the thread I made after going this route: