I have a pretty reliable thumber that I use regularily.
I just tried that pistol grip pictured on the first page,and it made me laugh.
My hand wont even do that.
Even if I could
get my thumb all the way up on the rim like that, since both my thumbs have been dislocated, I'm sure my thumb would separate when I threw the disc, and probably fly farther than the shot.
I guess it doesnt help that I use a Gazelle or Eagle X for thumbers, but my grip is comfortable with the thumb hooked into the rim.
My index finger parallels my thumb on the other side of the flight plate, so the pressure from the tip of my thumb press's against the first joint down from the fingernail on the other side.
This allows me to align the disc by feel, then its clamped down prior to launch.
The outside rim cradles into the webbing below the thumb, and sits on the first joint on my middle finger up front. All of this helps with alignment for me. Once I know the disc is always going to start at the same resting position, its easy to adjust the hyzer angle on release to change the roll rate as required.
Grip is subjective though. Whats not is the follow through. As stated before, never stop the motion of your arm suddenly when executing a thumber, you will do serious damage to your shoulder.
Always follow through like a windmill, letting the energy release from your arm gradually. This is the single most important thing if you want to pull this shot out a whole bunch of times in a round.