That mythbusters episode did indeed do a small mockup and it seemed to work. They also tried various materials but I forget which worked best..
Then they tried it the way the ancient greeks were supposed to do it and it failed. They couldn't get everone to focus in exactly the same spot.
Lastly the tried the large scale mounted version and it did start to heat up the boat, but it look so long and did get hot enough to cause any problems.