A thimble makes an excellent thumbstrap substitute.
Some time back when I was playing Super Mario 64 DS, sheer frustration drove
me to a sewing shop where I found this thimble that looked sort-of like a
fingerpick for an acoustic guitar.   (Not the old-fashioned thimble that
looks like a little bucket.)   Off-white thing, with "wings" to go around
and a pointy bit that sticks out past the tip of the thumb.   Turned out it
was made from heat-mouldable plastic.   It was easy for me to mould it to
fit my thumb perfectly, and to bend the pointy bit around the tip of my
thumb & get a good round contact surface.   The plastic isn't hard enough to
cause any scratching on the DS screen.   It's excellent for Super Mario and
Metroid Hunters.