There is no specific end cap for a DD omega. The Omega is a two piece design meant to be a drop in free floating replacement for standard hand guards.
The reason why the YHM rail has an end cap available is simply because it is a one piece, tubular design that requires complete disassembly of the barrel group to install. Since the front sight needs to come off anyway, the manufacturer decided to offer a cap to close up the end.
In fact YHM is the only FF rail that I can think of that has an end cap as an accessory. Most rails are left open at the front. Why add more weight?