自制小自拍反射鏡

My mobile phone does not have a mirror but I want to make one small reflective mirror and stick on my mobile phone for taking photos.

I've tried to use 銀色紐扣 but it is too difficult to stick on the phone.
I've tried to use 銀色2兩腳釘 but their reflection is not so clear.

In some Taiwan website, there is a thing called 自拍時必用的圓形反光貼紙.
However, I live in HK. Don't know where I can buy it or how I can buy it from Taiwan.

Any suggestion? (I want some not-too-expensive way).

Thanks.