There are many hooks on Thinkiverse, but I noticed this Headphone Hook by Misguided had employed a curve on the hanging tab to add strength to what otherwise would be it's weakness, a 90 Deg bend with a flat tab. A few minutes in Sketchup enabled a modification of the design to produce a very useful door hook!
I narrowed the "U" so the door would close, even with the hook over the top (3mm gap). I also added two ridges to the "U" to grip the door and give the hook a snug fit. The printed item looks well and is very strong when printed in ABS plastic.
I've seen hooks on sale in the shops but they tend to be for narrower doors. I was able to print a hook that fitted our internal doors perfectly (45mm - 1 3/4"). I've placed a bunch of sizes in the Sketchup file and posted it to Thingiverse .