I recently changed jobs and have to park outside in the hot Atlanta sun, so suddenly I care about getting a good sunshade for my i3's windshield.
This turned out to be harder than expected. I found no i3-specific guidance online (including not here), so I have written this up as help for the next i3 owner.
$12 sunshade from auto parts store: flimsy and too small and doesn't cover the whole windshield. Would probably sag to the point of uselessness after a week. I returned it after a single day.
$50 sunshade from Heatshieldstore.com: fitted to i3's windshield, but it's one big ROLL UP shield, not accordion. A real pain to handle. Also pretty thin and seemed likely to sag after a while. Returned, and they refunded the original shipping cost too!
$50 sunshade from WeatherTech: looked like the same at Heatshieldstore, so I didn't order it.
$90 sunshade from CoverTech: heavy stock, accordion style, fits the i3 windshield, yay! Finally found one that fits. HUGE problem though: it's two pieces! The product description absolutely does not say that. Still it's the best I could find. The sunshade itself is $68, the bag (for winter) is $9, and with shipping the total was around $90. Expensive but dammit I just want to get on with my life here. Note that they don't ship quickly either, so be patient.
I'll add a photo of the CoverTech sunshield in the first comment.