Hi Mathew, I'm very sorry to hear about your theft but unfortunately It appears this problem isn't going to go away any time soon, I have also taken the same approach with an aftermarket cat and It does give me comfort to know the scrap value is pretty much zero ... let's just hope the cat thieves also know this if they try to steal them again ...
That being said this is the same supplier I looked at but the part numbers are different, these are also advertised for the Rx and are slightly cheaper, and as they are universal cat's I doubt theres any difference, the Magnaflow 400 ceramic was £30 less and is the same size, the Magnaflow 200 metal cat was £20 less and is slightly smaller, it is purported that some improvement in performance can be gained from the metal cat on a petrol engine, I'm not sure whether this would be the case on a hybrid system like the Rx though ...
Ceramic https://www.ebay.co.uk/itm/Magnaflow-400-Zeller-Ceramic-Catalytic-Converter-Lexus-Rx-2-1-8in-C4m/303656103323?fits=Car+Make%3ALexus|Model%3ARX|Cars+Type%3A400h&hash=item46b3506d9b:g:1m0AAOSwghddWDin
Metal https://www.ebay.co.uk/itm/Magnaflow-200-Zeller-Metal-Sport-Catalytic-Converter-Lexus-Rx-2-1-8in-M4a/303638783272?fits=Car+Make%3ALexus|Model%3ARX|Cars+Type%3A400h&hash=item46b2482528:g:7ekAAOSwtOVdZ-Ui