http://www.the-backup.com/
Have one of these. They are made of plastic or a polymer but they are sturdy enough for the job.
It did what it said it would do, but it might not be very practical.
If the foot of the bed and the side of the bed where it is installed is not facing the door, it will be unwieldy to get into play. While laying down, you will have to swing the gun across the bed or around the room to find a target. Not so easy with a fully loaded pump or auto. You will need to sit up to keep from sweeping your feet.
The pump will only have one shot before you need to get both hands on it and that will take sitting up.
Sitting up with a ready shotgun in your hand might not be the safest thing you can do either.
If you sit up first and put your legs over the side, the weapon will be blocked by your legs.
It is really only an advantage if the side of the bed with the weapon is pointing at the door and then it would be best if it were a light chopped double or auto.