You won't be able to control the tank temperature through the material being used. You might be able to increase or decrease the rate at which the tank changes to the ambient temperature but that's about it. What you can do, is make sure the tank stays in the shade which will help keep the temperature down but if you have high humidity, it's likely the tank will be fairly warm in or out of the shade.
The other options would be to use evaporative cooling which works poorly in high humidity environments, or use a commercial chiller, which is expensive and expensive to constantly run. It's basically like running a refrigerated air conditioning unit for the tank.
But they're pretty hardy fish and I don't think you'll have trouble keeping them in 90 °F (32 °C) water as long as you can keep the water quality good enough for them. And as most fish go, tilapia can live in some pretty nasty water, so I imagine you'll be ok with some reasonable filtration and husbandry. When water heats up it loses the ability to hold oxygen, so you might want some air pumps and air stones to maintain as much oxygen in the water as possible.