For deep sky imaging I (have to) use my Very Short Eye Piece Holder. Of course it is nice to be able to swap camera and eyepiece and that both are in focus. This proved to be impossible: my 25 mm Plössl eye piece had to be extended OUTSIDE the eyepiece holder to be able to focus.
To overcome this problem I have made a Parfocal Adapter. I used standard PVC tube and I assure you that I am rather happy with my tinkering!
The idea is to get perfect focus with your camera and then - without changing the focuser position - use the adapter to make your eye piece parfocal. After this the adapter has to be glued together for permanent 'parfocalling'.
Click any of the following thumbnails and a larger image with explanation will pop up.
I hope this helps you if you have the same or an identical problem with 'parfocalling'.
The latest improvement: a friend-with-a-lathe made me a nice aluminium extension tube: