I am thinking the same as MissPriss ....
Yes, it would be nice to personalize it by making/adapting it yourself.
However, with it being for use in water, safety is paramount.
I would not want to risk the adaptation being a downfall to the end design or results,
and possibly creating a danger.
Instead, make a fun wrap for your grandson for before/after!
He'll love it because it came from you.