The problem is that the RDB is so new that no one has come out with after-market accessories yet. The RDB recoil pad is a rubber pad that slips over the rear of the stock and is glued in place. You can try making an extension of the length you need by screwing a block of material to the rear of the stock with a plate at the rear that overhangs the extension block by about i/8" or so. Use the rear of the RDB stock without the rubber pad as a template for the plate so the rubber pad can grip the new plate as it grips the existing stock. The extension block should be a little smaller than the plate.
Or wait for someone to come out with a slip-on stock extension.
I've taken the pad off so I know how it's attached. What I was hoping was maybe someone had already pulled like a Limbsaver pad off their (AR/Rem 700/Musket/Toaster/Whatever) and said "Oh hey, this is a great fit!" I already tried a Magpul pad I had but no dice.
I think you're right in that someone is going to have to fabricate a custom part for it to make it work.
Even if there were a slip on, I don't see it staying there. there is is a rim but not much to hold on to.
I would be looking st something to utilize the two bolts. Buy longer bolts.
3D printing anyone?
Keltec showed off the RDB Tactical or whatever it was called athe SHOT this year and one of the features was an adjustable stock that could add I think an inch of LOP. They said it will be available as an aftermarket upgrade part from Keltec later this year, so I may hold out for that. Honestly it shoots great as is, having a little more LOP is more of a want, but not a necessity for me.