E.M.
E.
I had this same problem and this is what I did:
I went to a furniture upholster and bought very firm foam about 8 inches wide and high and a little longer then my baby. I covered by gluing a very thin fabric onto it. I made sure it was close enough on both sides of my son so he could not roll over. I can't remember if I had to wedge rolled up towels between the "bumpers" as I called them and the crib sides or not? The good thing was it worked like a charm, the bad side of course is the risk of having anything in the crib that could effect their breathing if they became pressed up against it. My feeling is at this age they have a inborn reflex that would automatically result in their moving their head away from the bumper or anything else obstructing their breathing. They have full control over their neck muscles at this stage and an innate preservation for survival.
I have two children, 7 and 12 years old. The 12 year old was the one I used this method with. He was three month premature and colicky ALL day. I guess maybe I used drastic measures to get some peace at night. My younger girl would climb out at 18 months from the crib , so with her I had to buy one of those tents that attached to the top of the crib to prevent her from escaping. It was see through mesh and shaped like a bubble so she could still stand up in her crib.
Because of my son's prematurity he had very poor oral muscle/sucking control. I also developed a binky with elastic so it would fit around my son's head. It was loose enough so he could spit it out if he wanted to but would keep it in place while he sucked instead of it falling out constantly in the car where you can't reach it to put it back in. I only used this in the car where I was able to supervise.