For a standard WPC siderail? - Genuine 3M double sided foam tape in 3cm width.
Protip - Don't remove the entire backing strip until the siderail is on the machine with the back screw and front bolt for the siderail in for correct alignment.
Peel off 5cm from one end and leave it hanging down, install the siderail with the screw and bolt in then remove the rest of the backing strip by pulling the remainder downwards.
That way the adhesive part of the tape isn't exposed to the air at all and it completely eliminates any potential alignment errors.