I am also concerned with the durability of the clear top and the heat transfer to the interior. Cars with metal roofs even convertables has insulation padding to reduce heat transfer to the inside of the car. With only the sliding shade as insulation, it would seem reasonable that the inside of the car will get hot.
The is a company in California, California Pop Tops, that makes an easy to install cover for the top, windshield, side windows and back window. I called the owner an asked if he makes one for the Smart. He said he did not at the moment, but if there was an interest he would make them.
Check out the website and look at the videos.
Car Covers, Car Sun Shades - California PopTop
Installation Demo: cable/DSL (51 seconds)
Removal Demo: cable/DSL (40 seconds)
If enough of us contact them, maybe he can gear up and make some on short notice. I told him I wanted to be first on the delivery list.
Other than wanting to buy a Pop Top for my Smart, I do not have any connection to the company.