I think lapping valves using one of those suction cup sticks is hard and there must be better ways.
So here is what I was thinking. I was thinking to get a really thin (medical or engine) hose that the valve stem will fit into as a male into a female and the hose will hold it tight. On the other side, I would stick a drill bit (that is on a drill) that is the same or similar size as the valve stem into the hose just like I did the valve. The hose should be as short as possible for accuracy and the only reason to use it because the drill mandrel will not reach the end of the valve. Then you spin the valve using the drill and hose rather than manually.
Can you verify if this idea is feasible and will not damage anything?