We used a kingsize for ourselves, had the cot next to it with the side taken off (bit of surgery required). As both mattresses on same level this worked a treat - cot mattress was like a small extension to main mattress in terms of access, comfort etc but we couldn't roll over onto him! As he got older we replaced the cot side, but left it down, then raised it, then gradually moved the cot closer and closer to the door..... until he was in his own room!
Worth a try?