I almost never disagree with Al (ALB)... but I always have (and probably always will) respectfully disagree with how he views airflow inside the engine bay.
The fan on the engine moves more air than you can imagine. The engine will always pull air from the engine compartment and discharge it to the bottom. Any hole in the engine bay (be it through the deck-lid, license plate hole, wheel-well hole, or firewall hole) is going to be a place for air to get in, not to let it out.
It's clear that something is happening with the air when the car isn't moving, and that the engine must be reingesting hot air. It's clear that the engine is going to radiate heat when it isn't moving (and when it is), and it's possible that at low RPMs, the fan isn't moving enough air to overcome a rise in the engine compartment temperature. Regardless, that fan is still moving more air than you think, and the heat isn't flowing out of the holes in the engine bay. Something else is happening in stop an go traffic.
Any time somebody posts a picture of a car without surround tins, we jump on him like a fat kid on cake, because we intuit that the air coming into the engine compartment should be as cool as possible. I'm pretty sure these surround tins make exactly zero difference when the car is moving down the road, and may even make the situation worse because the air under the car (when it's moving down the road at a decent clip) is not preheated-- because the vehicle is in motion, and the car is not riding on a bubble of air. Any hot air flowing out the bottom or off the exhaust is dozens of feet behind the car by as it moves along. It's when the car is stopped, or barely moving that those tins come into play.
I've tried the big honkin' holes in the firewall with big honkin' fans-- and it actually makes no difference under normal speeds, and makes the situation worse in stop and go traffic. I suspect this is because any air I'm getting from the front (firewall) side of the engine is hot air that has already been though the doghouse cooler. I also suspect that leaving a gap at the breastplate, or drawing air from the wheelwells is just more of the same-- hot air that the engine has already heated, rolling out from under the car when it's stopped. The standard A1 method of dumping the exhaust under the car, rather than out the back is madness of the first order.
What can be done? A lot of this is just baked in the cake-- a speedster engine compartment is a little bitty thing with it's air intake grill sitting in the worst place on a vehicle to try to draw in air (the low pressure zone at the back of the car). The hole behind the license plate it at least clean (not preheated) air, but this is an even lower pressure zone when the car is in motion, although I suspect it would be better when the vehicle is stopped or in stop and go traffic.
I plan (LORD willing) to get the car painted next winter. When I do, I plan to open up the deck lid as much as possible, probably removing all of the rain-tray apparatus. Will it be enough? Probably not, and it'll make driving in the rain a pretty thankless endeavor, but the alternative is to prop the hood open any time it gets hot, and that's pretty unsightly.
There isn't a great solution, which is why we try to optimize the configuration of the engine and deal with the symptoms of overheating. It isn't perfect, but until somebody builds a Baja Speedster, we deal with the limitations of what we've got.