If you want to keep the sun off the windshield, side glass, rear glass and the top use a California Pop Top. It is white and it attaches to the car using the mirrors, wipers and a adjustable tie in the back of the car. I have used for the past 7 years and it really keeps the cabin much cooler because it covers everything, including the top. The cover is light weight and folds up very small for storage (easily would fit up front). Here is a link to the site:
i3 uses a XL cover which is $79.99 and is made in the USA.