We got a cot from IKEA that converts to a cot bed when you take one side off, and then just took the side off, adjusted the mattress height to be flush with our mattress, and attached it to our bed. They are designed to be stable and bear weight with only three sides (as they convert into the day bed type cot bed) so was perfectly safe that way. We just pushed the cot up against the wall to secure it, but some people do actually rachet the beds together below the mattresses. I think it's much better value than the co sleeper cots as it's much more affordable, plus you use a standard cot mattress so it will last a long time. We used ours for a year. I know friends who had a co sleeper type cot and their lo's outgrew them around 4 months.