You need a firm unsagging mattress, pillows well out of the way, and light covers only. We also took the foam pad off the mattress. The most successful arrangement was taking one side rail off the crib, and wedging it against the bed with NO gaps - wired the frames together. The crib mattress was half an inch lower than the bed mattress. I could keep an arm around the baby, pull him/her in to nurse, not worry about dh rolling over, and have a little more room to sleep in.
Mrs VS
There actually is a product available that is the same thing as what you describe. A little bed that is minus one side that attaches onto one side of the parents' bed. My daughter had one for her baby. I had never seen one before. I would imagine they are expensive, though. My daughter and SIL have always spent a lot of money on their little girl.