Sometimes HDPE is continuously supported in channel, mostly small bore. Larger lines will have far smaller spans than metallic pipe, e.g. 1.3m for 100mm nb at 60 deg C.
Support attachementsare usually clamp type, either steel with a Tico liner, or made from plastic. You are looking for decent load distribution and no sharp edges or stress concentrations.
Have a look at this website
www.georgefischer.co.uk it has very helpful design data and advice,