If you can't get a proper traveller
track the first thing I would say is forget it. Even getting straight curtain runs done without the proper
track are hard.
OK if you insist on this idea I'll give you a few thoughts.
1) How much spare traveller
track do you currently have and is is it made up in sections? Why I ask this is you might not be able to do a circular
track but a rectangular one with corners. It would take a
bit of
track and you might have to buy/rent the corner pieces. What you would is bring the side pieces out ten feet you need. Add a corner at each side with the
track length being 20 feet across.
But you still have some roping issues also you haven't mentioned how you plan to suspend the
track.
Curtains of this size will have quite a weight.
2) Use plywood to make the circular shape. You still have problems on making a suitable
track. You might use like plastic
conduit screwed to the plywood for the rollers. Once agin roping it will be a problem but your pulley idea will work with a
bit of effort.
3) If you can't buy see if you can rent from one of the hire places you probably have a curtain specialist in your state.
I don't want to discourage you but without spending money this won't fly.