We measured carefully and bought a Kong 50 quart cooler. It fits just fine under the tonneau cover.
This is the cadillac (Genesis?) of coolers and is on the pricey side. It has plenty of insulation. It does not come with wheels and in fact has rubber feet which has the positive feature that it won't slide around, but also, you have to physically pick it up to move it (since it won't slide). It is also bear proof if that is a consideration for you.
We are very happy with it and no bears attacked the SC when we camped in Shenandoah National Park.
This is the cadillac (Genesis?) of coolers and is on the pricey side. It has plenty of insulation. It does not come with wheels and in fact has rubber feet which has the positive feature that it won't slide around, but also, you have to physically pick it up to move it (since it won't slide). It is also bear proof if that is a consideration for you.
We are very happy with it and no bears attacked the SC when we camped in Shenandoah National Park.