There are a few ways to make effective use of projection lenses on DSLRs and system compacts, the simple and the complex. I’m somewhere in-between.
My favourite interchangeable method is to make use of a relatively cheap Chinese M42x1mm to M52x0.75mm focussing helicoid, with the M42 end going into a AF confirming adapter for the camera.
At the M52 end I use a step-down ring, with the inner reamed down to fit the shaft of the projector lens. When the lens is aligned correctly in the step-down ring, the ring is glued to the shaft of the projector lens.
This then gives you a system where one focussing adapter can be used with multiple M52 mounted lenses.
Some lenses have shafts that limit their ability to reach infinity by this method, and I have used a pipe cutter to trim the shaft close to the last element, this way a more practical range can be achieved.