It is very easy to turn spey foregrips on the blank. Use a dowel or drill bit that is small enough to fit inside the rod's butt end that needs 4-10 layers of masking tape wrapped on it to take up the space to produce a snug, but not tight fit. The portion covered with masking tape ought to be around 3" long so the butt end is well supported from inside. Then simply chuck the drill bit or dowel in a drill or rod lathe (I prefer drill bits because they don't flex) and put the other end of the blank in a rod support (I also wrap a couple of layers of masking tape on the blank where it contacts the rod support to prevent blank damage and scratches) and turn on the power.
Since it only needs light pressure to sand the cork down and form the grip, it is very easy to control the amount you take off and it is also very easy to keep the rod blank from heating up where it rests in the rod support by going slow, turning off the drill of rod lathe after each pass of the sand paper, and checking your progress.
I use 1) 60 or 80 grit to cut the cork to shape; 2) 100 grit for final shaping; 3) 150 git to start smooting the cork; and 4) 220 grit to put the final finish on the cork. It only takes about 15-20 minutes to shape both foregrip and rear grip with this method.