I'm a recent convert to the SuperSharpie myself, having been a loyal, if only occasional, user of Lee Earle's design. My problem with boon-style gimmicks was the stability issue.
The reality is some of us have fleshier digits than others, such that boon-style gimmicks are much harder to use. Were it not for that, I would be perfectly happy with a painted thumbtack, blu-tack and a glued-on pencil lead.
That said, it seems to me that you should determine whether the SuperSharpie's instability is due to the adhesive or to your thumb's flesh (i.e. is the gimmick rocking on & off your skin, or is your skin just moving around with the gimmick?).
If it's the latter, there's nothing to do about it short of surgery. My SuperSharpie penmanship will always be sloppy because of that fleshiness, so you're not alone if that's the problem. I just write bigger letters & numbers under more cover.
But if it's the adhesive's tackiness that's the problem, you may already have tried double-sided Scotch tape. Also, the more you "blacken" the putty the way Chris Smith demonstrates, the tackier it will become (softer too, and more prone to leave residue, but oh well). Mine is very black and very tacky.
At the thumb-side, you may want to vigorously rub your thumb pad against your pants or a rough, sandpapery surface just before performing. That'll exfoliate any dead skin, sweat and/or oils that can interfere with adhesion.
More putty should never be the solution, though. It doesn't adhere more strongly, the more of it you use. Use as little as possible, regardless of your thumb condition. If a little won't stick, a lot more still won't stick.