We recently moved our DS over to his cotbed. Initially, we put him back in the moses basket in the cotbed so he had that closeness and warmth.
When he really did get too big for that (his head and feet practically touching at either end) (around 22-24 weeks) we moved him to the cotbed, but packed him in with a load of rolled up blankets on either side, below his face, so he couldn't roll over.
Over time (and as he's got better at rolling and self-supporting), we gradually ended up using fewer blankets on either side although we still have one on each side to support him/give him something to hold onto.
I started back to work a few weeks ago which coincided with a raft of coughs and colds and viruses so he started sleeping with us in our bed (he's now 8 months). Getting him back has been difficult as our bed is warmer, softer, has an all-night milk bar and mummy and daddy for comfort.
So we put a cellular blanket under his cotbed fitted sheet, raised the head of the mattress with a towel (all-night coughing not helping - we put him to sleep on his side with blankets supporting when he was coughing a lot - really helped), added another cellular blanket on top of the sheet, put a bunny teddy next to him to hold onto (he started liking this at around 7 months) and use a couple of moses basket flat sheets tucking him in on top of his sleeping bag (mimics our duvet on his legs and feels like a hug).
We've arrived at this after a lot of trial and error, but it works. Starting nursery and learning to nap without being fed to sleep has really helped him self-settle as well.
Maybe ask yourself if you would sleep in the cot (and if not, how would you make it nicer/comfier/warmer etc) would be a good starting point?
Also - so much is going on at that age. Our best sleep spell so far was around 4 months when he'd only wake up once for a feed (breastfed). Since then we've had to contend with illness, moving rooms, teething, weaning to solids, starting nursery, learning to crawl, crawling, learning to stand etc etc
If you had that much going on in your life, you'd want a bit of comfort!
Good luck :)