Take one eyepiece cover and cut a hole in it. Make some slight adjustments to a iPhone hard case cover and stick the two together. Cost - eyecup came with scope and hard covers from as little as £0.99 - I spent £2.99 for a nice one!
My iPhone simply slides in and out with ease and when the eyecup is simply slotted over the eyepiece it is centred on the eyepiece.
Here is a test shot with the eyepiece set at lower end x25.
And here's the same scene using the phone's own zoom and the eyepiece still on x25.