Well I could put the cloth directly above them, so that the sun is mostly only blocked when the sun is overhead.
And here is one example of members of the genus in full, unadulterated sunlight:
This is Ctsm macrocarpum; photo taken in Trinidad & Tobago. As you can see, they are growing completely unprotected, on the top of telephone poles (I've seen Fred Clark share a similar image in one of his talks).
I don't imagine ALL ctsm are this sun-tolerant, but the SVO caresheet says they do well up to 5000 FC. Sooo I'm thinking that using 60% shade to bring the light down to 4000 FC should be safe...