Crib Kerfuffle

I've been trying to transition my 5mo son from his cradle next to me to his crib in his room. I'm not really into the idea since I'm a paranoid nut and I typically watch him breathe all night, I know he needs to be in a bigger bed since he's very long and super active. Every time I put him down in there for a nap or at bed time, he wakes up and rolls around and doesn't want to sleep. That's whatever, we'll work on that, but I put him in there today just to hang out while I went to the bathroom and he flipped onto his tummy and got his leg stuck between the rungs!!!! He was screaming and scared, understandably so, and I had to really push to get his leg out. I know the APA is anti-bumpers, but how can I put him in the crib if he keeps flipping and getting stuck?? Unfortunately, my husband and I are teachers so we aren't made of money, getting a new crib isn't happening. Any ideas on how to prevent my son from getting stuck??