Just buy some nylon tube, split length wise to fit.
https://www.advancedfluidsolutions.co.uk/16mm-x-13mm-natural-metric-flexible-nylon-tubing-bs5409-3056-p.asp
or possibly
https://www.overclockers.co.uk/ek-water-blocks-ek-tube-zmt-matte-black-16-10-mm-1m-wc-535-ek.html
As I mentioned above I'd prefer not to reduce the width of the rack as I have other panniers I use that fit it.
If split length wise just remove when not needed? That will fulfil your requirement not to change the size of the rack permanently.
Like so, the red foam can be removed as it has a longitudinal split.
@grundy started
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Just buy some nylon tube, split length wise to fit.
https://www.advancedfluidsolutions.co.uk/16mm-x-13mm-natural-metric-flexible-nylon-tubing-bs5409-3056-p.asp
or possibly
https://www.overclockers.co.uk/ek-water-blocks-ek-tube-zmt-matte-black-16-10-mm-1m-wc-535-ek.html