Place one of our floor or table lamps, and see if the translucency i the shade works for you.
I'm not sure what you mean by seamlessly? It will usually show the geometry behind he fabric.
(You could sent the framing behind the geometry to not cast shadows)
Also, you could make the fabric itself the light source, rather than the light behind it
Also, depending on how many lights you have, you should see if things get better with more rendering passes.