Beeswax alone isn't good for waxing up glass pieces on a glass easel, but combining it with Venice Turpentine makes a secure fixing material.
250gm block of beeswax
2 teaspoons of Venice Turpentine
Bring the two elements just to a simmer, stir it well and cool.
This produces a tacky material that when warmed in the hand sticks to the glass and the easel. When it cools it sticks firmly.
Use a palette knife to loosen the wax from the easel.