My suggestion is to make one with sides composed of 1/2" plate, spaced
apart (ID) a width that equals the width of your dipper, plus 3/4". The
3/4" (2 x 3/8") allows for 2 pieces of 3/8" plate to wrap along the sides
of the dipper between the 2 bkt pivot holes. This assumes that both
bkt pin pivot tubes stand proud of the dipper by at least 3/8".
You can also use the main bkt pivot hole as your thumb pivot, but that will
require more work to make a special extra-long greasable pin. You can make
a wider thumb this way, however.
Shape your side plates any way you want, with or without teeth. When
perpendicular to the dipper, your thumb should clear the main boom by at
least 1/2" at full-retract of the dipper.
For the
L35, I would not use any pin smaller than 1" diameter, even for
mechanical (not hyd) thumb.