The $12 door latch jig I should have made 15 years ago

I spent 6 hours last Saturday hanging a set of cabinet doors in a kitchen in Lansing because I kept fighting with a cheap plastic hinge jig that slipped on me. My dad, who has been trim carpentry for 40 years, watched me cuss for about 20 minutes and then said, "You know you could just make one of those out of scrap maple in a half hour." He was right, and I felt like a fool. I built a simple L-shaped jig from a piece of hard maple I had sitting in the shop, drilled a few holes at the right spacing, and knocked out the next 8 doors in about 45 minutes. Sometimes the fancy store-bought tool isn't the answer, and I think we all know a guy who would rather buy than build. What's the one jig you wish you had made years earlier instead of buying?