This is the sort of problem where someone who knows about bow-drilling could probably easily tell you what needs changing if they could just see you trying to do it - so any chance of you going to a meet where that might happen?
Re limpet shells - I agree with Ged - they make it so much easier than using a piece of wood with a cavity cut into it but .... when I tried recently I found that you needed TWO limpet shells (with one inside the other) as the friction causes them to heat up and one would not be enough to avoid burning your hand (well ... that was my expererindce anyway)
One day I might grow up - but I hope not
Say 'yes' unless there's a good reason to the contrary