[PeterM]

Stephen that is an excellent job well done.

As far as insulation the first thing I did was to paint the roof with 2 coats of silver ormonoid bitumen paint from Bunnings. This has had a huge impact on reducing the heat transfer into the observatory. I redo it about every 3 years. Another idea I use in summer is to have sunshade cloth on the Northern and Western walls. From Bunnings you could use the Cooleroo outdoor sunshade roll up blinds. They retract on a spring, so by bringing them to ground on an angle to the shed wall you allow for cooling from the material and the air gap. In winter you simply retract them.
Even if you line the walls and insulate them I think you will find the sunshade keeps the walls and the inside significantly cooler.
I also have a small caravan air vent ontop of the roll off roof, this helps significantly.
My Observatory is no hotter inside than outside on real stinker days and is often cooler by a few degrees.

PeterM.