While I am not familiar with the vintage Rainbow Rider I have restored a few keel-less delta kites in my day.
What I do is locate the holes in the kite sail where the original string passed through the sail to the spar. I will then cut a piece of string at least 10 to 12 inches longer than the distance between the two holes thus creating a very slack line that will appear the same shape as a keel. After securing the ends of the line to the spar I then cut another loop of string about 8 to 10 inches long and loop that onto the bridle line thus creating an adjustable tow point. I then adjust the tow point to the best angle for the wind condition I am flying in.
It's fairly easy and I have done this to about 20 kites and they fly like champs.
Hope you enjoy the kite flying season!!
- Judith Imhoff and Controlled Crash like this