Getting heat away from the LED is going to be a key factor for the long term survival of the LED. Even a 1W LED produces enough heat to self destruct in a relatively short time. You will need the LED mounted on a heatsink whatever you do. The LED you linked to is already mounted on a thermally conducting plate - but it really needs a way for the heat to escape. Mounting to a larger heatsink is usual - which is what the slots are for.
You don't necessarily need air flow inside the fixture, but it would be good if there was a way for the heat to be taken away from the LED. A big strip of metal to extend the heatsink mass will help and if it can find its way to the outside air, then that would be very beneficial. Perhaps you could make it a feature. If a large enough area in itself, it may not need to be finned - because that's all the fins do: increase the surface area to the ambient air.
You are spot on with asking this question, though. Heat is the number one issue with electronics.