This is a tool for drilling 1/8" pilot holes at 45-degree angles. I designed it for vertical use, but I guess it would work on any other surface too.
There are two visual aids to help you line up the guide tube with where you want your hole to be. If you make a mark 1cm below your hole center, that will be visible through the diamond-shape cutout in the middle of the part. A mark 1 inch below your hole center will line up with the bottom of the part. And a straight vertical line will go through the top and bottom cutouts.
Designed in Autodesk Fusion 360 (.f3d archive is included)
I printed this on a Replicator 2 at low quality settings (works just as well and faster). No support/raft necessary. Print it in the orientation in the file, with the part that will press against the wall at the top, so that it is as flat as possible. Takes about half an hour to print.
With my printer it does tend to lift up a tiny bit at the top corners, but a few seconds of rubbing on 200-grid sandpaper will flatten it out.