Our 4.5mo DS will only nap in the sling. I'm on the point of thinking that the only way of getting him to sleep elsewhere will involve significant amount of crying, but could probably just about cope for another month or so if there was a big enough chance that he'd just grow out of it. If you had a sling-only napper, when did they grow out of it?
Things we have tried include: white noise, dummy, blackout blinds, singing the same song each time he falls asleep in the sling and singing it when trying to get him to sleep in the cot, cranial osteopathy, various forms of attempts at swaddling etc., warming moses basket/cot with hot water bottle, experimenting with timings of naps, and probably lots of other things besides. I've read The No Cry Nap Solution but think I've exhausted the ideas in there. At the moment it takes most of the things above plus half an hour of sshing him crying etc. to get him to sleep for maybe 10-20 minutes in the cot. He doesn't sleep in the pram, car or swing - just cries, and if I feed him to sleep, never sleeps for more than 10 minutes. Don't think I've got the resilience to do e.g. ssh/pat or PUPD properly without support, so wondering about going the sleep clinic/consultant route (esp as we have issues with nighttime sleep too).