If you are still sliding forward, the seat wasn't made correctly. I have had at least four or five brands of custom seats and the RDL is by far the best for me. For whatever reason they don't like to raise the front of the seat enough. I guess they think it will be uncomfortable. When I had my RDL built I specifically asked them to make the front higher to keep me from sliding forward - a common issue on the FJR. They came close. After riding it for a 1,000 miles or so I still sent it back to have the front raised more. Voila! Perfect. Now, 11 years later that seat is a bit more worn and the front raised part has sunk just slightly as I guess the foam has compressed. It's still fine.
I have acquired two other used RDLs since I have too many freaking FJRs. On one the forward slide is there no doubt. On the other, it's just there a little bit. Still, those used RDLs are still more comfortable than any other seat I have tried.