So you’ll need someone to do custom work. I think all of the following do a cage with a frenum attachment, I’ve not seen anyone cater for a RPA, but you could ask them. Have a look at: Steelworxx Badass Workroom Rigid Chastity Evotion Wearables Male Chastity Now
I've previously modified a Contender to work with a RPA. It's simply a matter of mounting the lock and pin atop the cage instead of below the cage. And a couple customer's have used the Glans Armor 2 with their RPA's as well.