For super-slim SMT designs, this custom-made, 2x20 short SMT female header can be used to plug a Raspberry Pi HAT onto any modern Raspberry Pi computer with 2x20 headers 

It's harder to use than the Thru-Hole type headers, because you have to do some SMT soldering, but for the slickest, slimmest fit, this header is recommended!

Note that because the connectors go out to the sides, you can have holes in the PCB to allow connection from either direction - that is, the pin headers could go through PCB. You can see this in the photos of the Voice Bonnet, the pin ends are visible through the SMT header holes.