So I want a bottle with a vent on the top and a "recovery' type cap. The bottle I was looking at (the one that might fit) is 20oz capacity. I worry that a full to the brim radiator will discharge more than 20 oz and spit coolant out the vent. Valid concern?

Also not sure about my cap. I ran the car with a Stant lever cap (model 331) and when I shut it down a bunch of coolant puked out but the upper radiator hose was not hard (no pressure) Maybe it had relieved the pressure before I squeezed the hose? Not sure.