I actually build a physical room and place a SceneCaptureCube actor and then right click the CubeRenderTarget to create a static cubemap. Should be perfect.
Previously I used a free photoshop plugin called Flexify2 that could take a 6 sided cube layout and convert it to the equirectangular format, but its a pain and I would suggest building the room, even if it just means placing your 6 textures on a box mesh. Faster iteration and less room for error that way IMO.
also this came up yesterday: to support non uniform scale such as double wide rooms, in addition to scaling the UVs, you need to do the inverse scale on the CameraVector in the material function after the transform. Will probably add that to the function in the future.