I use foam which is used for isolation under laminated floors. It comes in a big roll. It is thin but strong. because it is foam, it is very fexible and does not affect your shaft in any way, exept for the weight. I cut ribbons of it, as long as the shaft is. I cut them on the weight I want and weigh hem with a digital weighscale. To give you an idea: a width of 3/5 inch weighs around the 40 grains.It is very easy to make the weight you are looking for. I put a wire through the shaft, knot it to the ribbon and pull the ribbon with the wire trough the shaft. When it is on its place, I cut the wire. I spray the ribbon with teflon spray first, to make sure it goes in easy and doesn't stuck somewhere.
The foam material doesn't change your spine and is very cheap. After a few 100 shots it creeps a little bit forwards in the shaft; let's say 2 to 4 inches. It does not push your nocks out after the shot.
Hope this helps!
It works great for me and I am adding 30 to 45 grains in this way.