I like this build. Thought the author did alot of work for the one particular drill to work with the build. Oh and used a CNC to cut it out. Oh and there are springs in the build (you just don’t see them in the pictures).
http://www.instructables.com/id/Drill-Perpendicular/
Oh if you don’t want to build this. You can buy one (look for drill guide) for 30-50 dollars.