Depending on the application, it could be as simple as creating a "box" in the objects/shapes folder. Change the dimensions of the box to match the desired cyc dimensions. The depth could be as little as 1mm. Create and apply a material and set a transparent amount to your desire. There are several other things you can change, the the above will get you started.

Thanks

I came to the forum with much the same question. I tried the suggestion of building a transparent box, but I find the results rather unconvincing. The moment I have sufficient transparency to show the light from behind the cyc, I also see sharply and clearly anything else behind the cyc. I can add a 'frost' to the material to blur those details slightly, but it's not really the same as a backlit cyc as I know it. Are there any further tricks I can use for a more convincing result?