This is actually a very old answer to the problem and is referred to in a number of texts from the late 18th century. Usually it was limewash applied onto slate roofs above attics that were used as sleeping quarters for farm staff in Southern England.
I'm not sure how effective it would be in Florida heat and whether it would stick to your roof would depend on the roofing material.
I use limewash on the (stone) house exterior and it is very cheap to make. I make limewash from lime putty (pour a bag of lime into a plastic domestic trash bin (wear a mask!), add lots of water and mix it as well as you can. After a few weeks it forms a cheese-like putty under a layer of clear liquid).
But you can make it straight from powdered hydrated lime. Not all outlets recognise the difference between hydrated lime and hydraulic lime, so make sure you get the right one. Be careful handling it and wear mask, eye protection and gloves.
I add a splash of linseed oil to my limewash to make it stick better and be less likely to rub off on people's clothes.
I've not used it on a roof, but it does stick to slates quite well. It gradually wears off in the rain, but where it was used in the past, they would have reapplied it most years until a thick reflective layer built up. This practice would probably also have helped waterproof old roofs and stabilise loose slates, so added benefits for those sleeping in the roof space. At that time lime and it's products were used widely on farms, so it was a readily accessible material.