I was in the middle of my latest Y6 build and realized that the GPS mast I had was not suitable to carry both my GPS and magnetometer. I do not have the latest 3DR uBlox GPS with compass kit (maybe this should be my next purchase).
Luckily, my neighbor just picked up a 3D printer recently so I decided to make a rough SolidWorks sketch of an adapter that would hold the GPS and compass and that would fit on top of the mast.
This is the finished product :) The HMC5883L has a little tab on the side so I modeled the piece to have a cut in the protrusion you see to the right of the adapter. It is just big enough for it to fit quite snugly. I have to say that having a printer at your disposal is incredible useful as a tinkerer, hobbyist, engineer, etc. I can see a time in the near future when there will be one in every household.