Christa - like Rose suggested, how about something like this:

The babies always sleep in here and it keeps them from falling between the mattress and headboard and we also know where they are and don't have to worry about rolling on to them. Once asleep., we can move them around too. After awhile of hard play at bedtime, they have been sleeping all night and now they are older and can come and go on their own.
Worth a try? It is just a $10 carpet condom.

One possible drawback Early morning wakeup!