Dave Wortley has released a useful Maxscript that can set up Camera Map projections of different aspect ratios.
“By default in standard 3dsmax the Camera Map Modifier has a projection aspect ratio the same as the camera/render resolution, this isn’t good if you need to project a portrait image on a landscape render. Whilst there are plugins/MCG/OSL solutions to this, my solution works on standard max, with no plugins needed to reload the scene. It even works on CameraMapPerPixel too,” states the description.
The developer states that the script is quite clever, so it will automatically find a texture on the object that the camera-map is projecting with that is on the same channel ID. All you need here is to select the camera. Do you need to change the aspect ratio of your render? Just re-run this script to fix all your projections.
You can get the script here.