I have added this to my shop at Tindie, so you can now buy it: https://www.tindie.com/products/deshipu/x-pad-20-buttons-shield-for-d1-mini/

It's the cheapest shield I have made so far. I am not entirely happy with it yet (the buttons are too small and I should have put the chip on the underside), but it's actually very useful, so why not let people get it. It goes well together with the official OLED shield: