Since 2015 Tillett P1 gives the highest level of rib protectors with improved focus on comfort without increasing thickness and reducing the feeling of the chassis. The rib protector has adjustable rear straps to adapt the many forms of bodies.
The size guide can be read longer down this site or you can use Tilletts P1 guide right here
Kartmagazine.com has made a good article about the Tillett P1. You can read it by clicking here.