The Apple MagSafe Battery, which unfortunately was discontinued, is still a brilliant little battery to boost your iPhone - and mine still works just fine. The only downside is, that it was only ever released in white, which looks a bit of an eyesore on a black phone.

So I modelled a case (which is held in place by strong double-sided tape).

Post-Printing

Double sided tape application

I would recommend putting a strong double sided tape on the battery itself, rather than the case, keeping the backing on while you test for a good fit, and going up to the left, top and right hand edges (but not the bottom), as indicated on the image.

Once you're happy with the fit, checking that the charge light lines up properly with the hole, and that your charging cable fits ok, remove the backing on the three pieces of tape and push into place.