Aaron, probably a couple of things to think about.
Sure you can spend a lot and line the walls and ceiling with proper insulation, or you can go with what you are thinking.
It will help but it wont get rid of all the heat.
As you know heat rises and needs an outlet so an exhaust fan or some other device to let the heat out would do a good job.
When I built my Observatory some time ago It was insulated to within a inch of its life however it still got hot.
I had a fan inside the Observatory that ran 24/7 to circulate the air and exhaust fan to suck it out.
With a tin shed as you have it heats up very quick however i cools just as quick when the Sun goes down.
Installing a fly like one dose over a tent is also a good idea as it lets the air flow in between the roof and the fly, but of course you need a decent gap between the roof and the fly
Leon
