I drive in the Western US and have had a good experience with my Heatshield for the windshield. It rolls up and has a Velcro closure – I toss it in the back seat when not using it.
I bought mine in 2014 – Heatshield hadn't yet worked out an exact fit but looking at pics on their website today it appears things are in better shape now.
https://www.heatshieldstore.com/car-win ... tchback-5d
Considering what your legislative environment allows, tint as many windows as you can, with as much IR protection as you can. The i3 has high IR gain from its huge windshield. I have SunTek CXP Series all around with the windshield at 80% transmittance – it's very good in cutting heat gain but I wish I could get even more.
Select a competent installer, the windshield in an i3 is not an easy one to do.
That said, keep in mind night driving. The i3 has a big A-pillar – combine that with a darkly tinted windshield and the driver's workload is increased at night. I often wish that, when driving at night, that I had no tint at all.