It is hard to know what to do, isn't it!!?? We had a cot for ds, so just had a fitted sheet on that. Then he was wrapped snuggly in a bunny rug, his feet placed at the bottom of the cot, and then a sheet or thin blanket tucket in tightly at the bottom and sides of the cot and over him. Basically so he couldn't wriggle down under the blankets.

Why don't you have a look at the sids website, I would imagine they would have their recommendations on there?