Some of the comments about the freelensing quickie we posted were deeply concerned with dust.
One way to solve the dust issue is to make a small step away from 100% free free lensing. The plunger lens is one way of doing so using a… well… a plunger 🙂
Another way of freelensing without concern for dust or flare is by building bellows. DC Watch has a nice video tutorial that you can follow along with a template PDF.
And if you feel comfy with slightly broken English, you can follow the instruction on the machine translated post from DC Watch.