Using Render Textures To Create a Cubemap

I want to use a render texture that I have created in a script as a cube map. It follows the horizontal format where each side is lined up all on the same level. The sides go X+, X-, Y+, Y-, Z+, Z-. However I can't assign this to my material that is using a cube map. I get the error that really doesn't make sense to me:

RenderTexture.Create failed: cube maps must be power of two and width must match height

If I export my rendered cubemap and then re-import it as a normal texture and do the cube map setup it does work as intended. However I don't want to do it this way because the conversion from render texture to Texture2D takes a very long time and I want this cubemap to be dynamic and capable of changing within a short amount of time.

So to recap all I want to do is use this render texture that I have created through script, as a cubemap for a skybox material that is using the cubemap preset. Is this something that can be done?