I bought the Len mattress protector sheet from IKEA for my baby's cot, only to discover that it is completely non-breathable and hence should not be placed under the baby's head. As the little one rotated in all possible (and impossible) directions during sleep, it didn't feel safe to use in the cot.
Not wanting to waste money, or time by going back to IKEA for a refund, I put the sheets in the cupboard for a later project.
Later on I took the little one swimming and realised that I had forgotten the extra plastic bag to put the wet swimming costumes in. Needless to say, the bag got all wet and so did everything around it.
This was when I got the idea to use the Len mattress protector as lining for a bag.
Simple design, but still fit for purpose.
Pattern: my own, based on the size of the mattress protection sheet
Fabric: flowers repurposed from a table runner and Len mattress protector from IKEA