Here is the best product for hot sun. I was also looking for a good easy on/off product that lasts long and does well in the sun for the roof of my 2010 GMC Savana van.
I bought Finish Kare 1000P. Goes on and off great lasts a long time and is a product designed specifically for hot temps.
It is great all around wax and is great for hot roofs and lasts forever. 4-6+ months.
I also use KAIO and KSG for the sides of my van but put the FK1000P on the roof and hood. I use a wheelchair so I can do the hood/sides and I get someone else to do the van roof. I wanted them to have the easiest to use product that lasts the longest and I found the FK.
Give it a try.
Finish Kare 1000P Hi-Temp Paste Wax, high melt polymer car wax, Finish care auto wax