I'll post pics of my custom made thumb brake lever that I had on, but don't remember .. They sell it for $55 I bet you could make your own?.
With having a prosthetic lower right leg, I have little or no feeling to operate the rear brake, I know Mick Doohan used a thumb brake to very.
The Thumb Brake introduced a new bike riding style that offers great But, for even greater results, IMA Special Parts has designed its own master cylinder, from billet in ERGAL alloy, with specific features made to exalt your braking feeling.
I might give building a thumb brake a go, at worst I can always .. just getting a quick look so i can dimension it up and then make my own, then.
The Kliktronic Gear-changer; Thumb brakes; The twin lever (Kilktronic K-Lever2) not fit every bike so you may have to have a small bracket made to suit your bike). Harley Davidson do there own version of this kit, see local Harley dealers.