Another possibility is the “Thumb Port” available from many flute dealers. I bought one for my Boehm/silver flute and it really helps.
Might be adaptable to a simple system flute without too much hassle.
JD
If I know what this is, it’s a plastic piece that snaps over the flute body, with a concave center that
fits the thumb. If my experience is typical, the piece will break/snap when one tries to snap it over
a conical D flute, the body being too wide. But it’s a good idea in principle and it would be nice
if something like this was available for our flutes.
This interesting device I found on a Cornelius Ward flute in the Edinburgh University Collection.
It assumes the 19th century grip and is presumably intended to make the location of the thumb more comfortable and secure. You play with the top of your thumb pressed into the centre of the target. It enables your right hand fingers to be straighter, and thus to spread further apart more easily.
The device locates between R1 and R2, but on the driver’s side of the flute. You can just see part of the ring-key for R1. The touch for the upper c key is clear on the left side of image. The three wires running behind the thumb plate are foot keys, operated by the left thumb.
If you haven’t tried out the 19th century grip, see:
http://www.mcgee-flutes.com/Rocksto_on_holding_the_flute.html
Terry