Remove the CSS safety line.
Attach a short pigtail to the front line where the CSS was formerly. It will just sit up there to hold the front line end where it belongs. But you could also use it as an alternate/back up front line leash point if you wanted.
Run a piece of kite line to an approximately 4m(?) long 2.5 or 3mm amsteel or dyneema leader line. Thread the leader line through the swivel center, through the bar and the CL fitting, just as the CSS line used to run.
You will end up with extra leader.
Launch the kite in gentle conditions, take up slack in the fifth and tie a loop into it at the CL fitting. The knot in the loop will hold tension and the loop is where you clip your leash.
Put a plastic ball behind the knot if you want, but it's not totally necessary.
I did it differently. I have cut CSS line short, so just a short pigtail remains on the front line (that's exactly what Lenten did, you can look it up on his old pictures from Cape Town). I have then attached the spare CSS line to a fifth line and put a plastic stopperball on it. With this I have put the fifth line through the swivel and that's it. You have a safety line on your fifth, a working swivel and working depower.
I run my 5th so it is the same length as the other 4 flying lines on the bar. It's nicer than having extra 5th when winding/unwinding and walking your lines. Fit each kite with an appropriate length nose line from the 5th attachment. Then you can use one bar on multiple kites.
How to get the nose line length: inflate kite and place it face down. Straighten out front pigtails and draw a line between them (just run a flying line across from one to the other). Nose line runs from 5th attachement down to where it bisects the line between front pigtails. That's a rough fit and you will have to put a few knots on it and put it up to get it dialled right in, ie. With the kite at 12 unhooked, no trim, the 5th should be just a touch slack compared to the steering lines.
If you run the 5th down through the swivel and bar as a safety your going to want to Make sure the first 4 meters on the bar end are hand friendly leader line. I put a bungee in the top of that part and a stopper ball set at he right spot for the biggest kite I intend to use with the bar. Set up this way it can also be used for relaunch/self launch in light wind/snow conditions.
Alternatively, you can leave the safety as is (one front line) and terminate the 5th at the swivel. Then it is static and doesn't need a leader portion, bungee, or stopper.