I've just measured the pipe I used. It's 15mm O/D.
https://www.ebay.co.uk/itm/Clear-PVC...VaPXIkYiqvtS0w
It is the O/D that counts, as you just push it into the outlet of the cooler and the outer edges make the seal. You should also get a short (5cm) length of garden hose to use as an adapter on the end of the return hose (that you removed from the cooler) to fit a funnel. Use this to add fluid.