You have several options :
-1- Make a small hole in the mantle of your mainsheet line and pull the core out thrrough this opening. You now have two tales.
-2- Pull the mantle back and cut of the exposed core and pull and line a 3mm back in in the mantle by its midsection. Stitch the line in place once the mantle is fully extended again, same for original core. (this is the setup I have and it is still holding well)
-3- Variation on -2- : extend mantle back after cutting away core. Open up the mantle next to were the core begins again. Run a small (3 mm) line through this opening and through the hollow mantle toewards the end of the mainsheet. Stitch the line into place as well as the core end. This approach is often simpler then -2- and rather easy to do at home. The mantle must be strong hower.
-4- Open up de mainsheet lijn and run a small diameter line through the mainsheet several times (from split point to other ending tale). Tie a figure 8 knot at the end of the small diameter line that comes out at the other tale. When it goes back and forth enough times then it will hold well.
-5- Take swift cord 6 mm (hollow) and run a 3 mm line inside it. Run it up the sheet meaning from split to where you will be holding the traveller sheet (or the cleat will). Just start the insert a little up from the tale end. Stitch both ends to eachother.
-6- Quick and dirty. Make a small bowline loop in the middle of a 3 mm line. Tie both ends of to the beam in such a way that the top of the bowline loop just won't touch the place where the traveller sheet exists the car. Now rig the traveller sheet through the car and through the bow line loop and terminate it with a figure 8 knot. The loop must be small enough to only allow passing of the traveller line. This works actually rather well, you only miss about 10-15 mm in sheeting relative to the perfect centre of the beam.
I hope this helps
Wouter