I am not sure if there is a diy somewhere, but I would do exactly what you said with a 3 inch pvc painted or you may even be able to find a piece that is black already. I see black pvc at lowes, not sure what people use it for in real life but that would save you a couple steps of sanding and painting the pvc. Make a thread once it is done I am guessing you are gonna use it to stick just your camera lens in?
Heidi