I used these from eBay to fit that exact model of Dually pegs you have without the offset extensions which are not required. You can get 1" clamps and use the offsets on your Roadster footpeg rails with another pair of Harley type mount pegs for a 3rd foot position option. All cheap as chips from the same Chinese supplier of the clamps below. All the same products can be found on Amazon for a similar price but International shipping may not be free like my eBay seller below. If you check out their footpegs for Harley mounts they will all fit your offsets. I have bought heaps of sets from them and they are identical in quality to Kuryakyn who probably get these made by the same Chinese factorys.
Alternatively other clamps will be available or get a set of Fehling 1.5" (38mm) front crash/tipover bars
Fehling
Crashbar Fehling Triumph Rocket III Roadster 10-16 silver
These are actually Fehling too just sold as 'US Custom' in Germany on eBay.
Aluminum Foot Peg Footrest Clamps for Harley Honda Yamaha Kawasaki 1 1/4" 32MM | eBay