Posted a lot about this lately, apologies! Short story is dd (10 months) is struggling to adjust to her cot having previously been in the next to me. As a middle ground I've bought her a travel cot as it's a bit more similar to the next to me than a wooden cot with bars.
As she'll be using it as a main bed for a while I also got a separate mattress. It was from Amazon and paired alongside the travel cot I bought as being suitable/compatible with the model. I noticed that while it does fit and the dimensions seem right, she is able to create a small gap between the mattress and the bottom of the cot if she pushes her head against it. Not big enough to get trapped down or anything but enough so her head is at a slight angle. Because she has always slept with her head butted against the top of the next to me I suspect she's doing the same in the travel cot.
Is this ok? I've spent loads of time looking into whether travel cots are suitable as permanent beds and got all the recommended stuff (separate mattress etc) but I'm still worrying.