Cot mattress - gap between mattress and cot

[acquiescence]

I ordered a new mattress for a second hand cot and am worried it is the wrong size- it doesn't fit snugly to the bars, should it? I have read that there should be a maximum of a 4 cm gap and I think there is about this. I ordered a 56x118cm which is what I measured it as and am wondering if it should be a 60x120.

I'm a bit clueless- could I just wedge it in with a rolled up towel or should i change it?

If you are not comfortable with it then I'd get it sorted. Personally I use my own judgement and common sense. I use an Ikea cot with one side off as a cosleeper. It means I push the mattress perfectly flush with mine but this opens a 15cm gap on the far side. I put small oblong cushions into the gap and they fit perfectly. I know it's not the 'approved' safety Susan method but a newborn does not roll and there is no way the baby would get stuck even if it could roll.