I made one of those "redneck" air conditioners for my MGB-GT and it would not keep up with the heat load. There simply wasn’t enough heat transfer by just blowing air through the box over the top of the ice. I even tried to baffle the air flow so that it had a longer path to travel before exiting the other side of the lid. I ducted the the exit with a 3 in. plastic plumbing elbow.