What Pac said. Top off the rad neck to the brim of the inside lip where the cap seats. You're wanting to get all the air out of there. No magic just pick the type/shape/color of bottle that suits you/looks good. I have a 2 liter plastic pepsi bottle on a DD w a hose to the bottom, a lever stant recovery cap, works fine. When an inventor patented it (then sold it to the car companies) it's selling point was that the elim of air pockets gave more uniform cooling and lowered temps as much as 10 deg (F) and less oxygen from losing the pockets slowed rust development