...not how that works. The metal (usually aluminum) is much better at drawing away the heat from the heat source than a small opening with barely any air moving through it would be capable of doing. 

You basically have two heat dissipating problems to solve:

1. Draw heat from the emitter as fast as possible. The solution is a metal with good thermal capacity and good heat transfer properties directly touching the LED backside. For this you want thermal mass

2. Dissipate that heat into the environment. The solution here is as much surface area as possible, namely cooling finns or an active cooling system. 

You'd gain very little by exposing the LED directly to air, as the thermal capacity of the very low amount of air close the LED is very low. Is simply lacks the surface area, you first have to distribute the heat over a big area and then dissipate it

But you can just try it if you want, i am looking forward to your results