Good luck with that.
The advent of 4" cordless angle grinders have reduced the value of steel cable, padlocks, etc to nil.
Add a cordless sawzall, and exhaust pipes are easy to cut as butter. Are the thieves unbolting them where you are? Its a cut-and-go thing here. Two cuts takes less than 60 seconds, and no dirty hands.
Around here, the CONTENTS of the converter may fetch ~$15 a pound. Selling them isnt hard, and any ID numbers etched in are easily obfuscated.
This is on our local Facebook marketplace, you will see similar in other online sales forums.
https://www.facebook.com/marketplace/item/666168434432580Like AC condensers, sooner or later the laws will change to make cat's hard to sell. But for now...

An alarm is the best idea, providing there is someone to respond to it.
Alter your cables so that attaches to a removeable pin, like on trailer emergency brakes, would sound it when the cat was removed. Or if the cable was positioned to be pulled on to get to the converter, possibly sooner!
https://m.media-amazon.com/images/I/61KEfOUUO3L._AC_SL1500_.jpghttps://www.amazon.com/Dumble-Trailer-Breakaway-Switch-Assembly/dp/B07G2RLY2C/Or live in the country with a big driveway, security cameras and weapons. Security through obscurity.

Sorry to hear of your troubles.