If you're one of the ones that missed out on getting the factory sun shades for your rear windows, here is a possible source to get them.
I was doing a search for rear sun shade options for my C-HR. I stumbled across this company which apparently launched last year on Kickstarter; Quiksnap. They produce side window sun shades for various cars which snap in the same was as the BMW 1 series factory sun shades. I just ordered a set for my C-HR and will report back on how well it works; especially for the rear side windows with the funky window frame contours.
They don't make anything currently for the 1 series but apparently their website indicates they will do custom jobs. I think if there is enough of you all that ask for this, they'll make them at a reasonable price. The company from what I see is based in California so if they need a test car for them to make measurements, I'm sure there is a member here that would be willing to let the company use their car.
https://quiksnap.us/
Good luck and I'll report back when I get the ones for my C-HR to let you know how it works out.