The cork can be made of any number of materials. It is more a matter of finding something that fits snugly, wonít come loose and is easy to fit into the flute. I have used a cork from a wine bottle, wooden dowels and a rubber stopper. Regardless of the material, the face of the cork should be about one embouchure diameter from the center of the embouchure; you may way to experiment with this to get the best sound.
My preferred cork material is, not surprisingly, actual cork. It looks professional in the end of the flute and doesnít swell or shrink and come loose when moist. This is what I used for my favorite G flute. The problem with cork is I havenít found source that doesnít need modification for it to fit 1/2 schedule 40 PVC. A standard wine cork needs to be cut down to fit with a plug cutter. This was difficult, labor intensive and frustrating. I also couldnít find any workable cork locally or sufficiently thick cork pads to cut plugs from
So, I tried wooden dowels. With a bit of a bevel on the end for easy entry and some sanding, a 5/8 inch diameter dowel fits well. I inserted the dowel to the correct depth I then cut off the dowel and smoothed the ends of the pipe and dowel to one smooth surface on the disc sander. This worked well for a while, but eventually moisture caused the dowel to loosen and fall out of the PVC.
This brought me to the rubber stopper from my local hardware store. The stoppers are tapered but to big to fit in the pipe without modification. To install a rubber stopper, I insert it as far as it will go, then cut of the excess with a wide chisel using a smooth slicing motion. This works fairly well, but isnít as clean or nice looking as actual cork.
For 1/2 inch CPVC it is probably easier to find cork stoppers at a craft store of the correct size (their were non large enough to fit the 1/2 inch PVC pipe.) I havenít tried to make a cork of either actual cork or the rubber stopper method for this type of pipe, so I donít know how well they work or how hard it actually is to find suitable material. Dowels are much more difficult to use for the 1/2 inch CPVC since no standard sizes fit. I had to turn down a dowel on a the lathe to get it to fit properly (although it hasnít loosened like some of the dowels in my PVC flutes.)