when I first read through this I thought that maybe the person in the shop was talking about woollen underlay type things, which add a layer of fluffiness to the mattress that could pose a risk. Maybe the heats getting to me...

We have a boori cot, which has it's own special sizing, and I've really like the Living Textile mattress protectors, because they fit so snugly over the mattress, they're really like an extra sheet IMO. I assume that they do different sizes to fit different makes. They also do a great job of protecting, ours have been well tested with a couple of seriously gross gastro events, and done the job well.