You will need a lens with a focal length of 12.5 feet or 3810mm. That's quite a lot more than a 50mm lens!
It might be worth contacting a local optician to see if it's possible to get a +0.26 Dioptre lens element (or possibly +0.25 Dioptre).
Another source of simple lens elements is cheap reading glasses. I don't know if they go down to 0.25 Dioptres though. I think they are normally in 0.5 Dioptre increments. Might be worth a look though.