You don't have to do a full block on the bottom. Just like NASCAR...you have to determine the opening by either using calculation or by trial/error methods. You may want to consider building some ducting so the air that does enter the opening gits directed straight to the cooling fins and doesn't git detoured somewhere else.
I would install an under the hood thermometer (digital meat thermometer would work) to see what temps you are gitting under there. I would establish a base line before you start blocking so you can see what's going on under there first.
