For mine, I used stainless steel wire rope woven all across the underside of the vehicle in a manner that would make it impossible to remove a catalytic converter if they cut it off without also having to cut most of the wire rope. This method prevents heat buildup that could occur if you use a steel or aluminum plate to conceal it while adding enough complexity to the removal that you make it more likely that they are discovered in the process. Most of these thieves are trying to get in and out ASAP.

So far it has worked. That is either because the vehicle has not been targeted (most likely case) or because the thieves saw who may have targeted it realized that there were easier vehicles to hit parked nearby (not unlikely but certainly less likely).