Buy a hollow pool noodle and split it lengthwise, some further modification may be required
for example
https://www.amazon.co.uk/Noodle-Hollow-Swimming-Rehabilitation-Children/dp/B07P5YBG56
It's easier to buy the pipe lagging material that's already split along the length. Then gaffer tape and in my experience it's unlikely to get stolen.
@Airhead started
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Buy a hollow pool noodle and split it lengthwise, some further modification may be required
for example
https://www.amazon.co.uk/Noodle-Hollow-Swimming-Rehabilitation-Children/dp/B07P5YBG56