Long story short, due to diabetic neuropathy I was having real issues with a repeatable bow grip.
I had a long discussion with my physical therapist. She checked my hands and made a suggestion.
This is working wonders;
Mueller wrist stabilizer (http://www.drugstore.com/products/prod.asp?pid=414072&catid=184148&aid=338666&aparam=414072)
It makes my thumb not put pressure on the side of the grip and forces the same hold each time.
Just an FYI, for those with similar issues.
You might be on to something. I always have to remind myself to not put thumb pressure on the handle.
It seems to hold my thumb right in place with no pressure on grip. The brace has 2 plastic stays supporting the thumb.