The maximum depth of the fore end for air rifles was increased this year from 90 to 120 mm, although a palm rest is not permitted on air rifle, a palm rest is a "handle" to hold which is fixed in the accessory rail, BUT, an extension to the fore end is permitted, and its near enough the same thing, just no anatomical features and it forms the same width as the for end, and in line with the fore end, and it serves the same purpose!

So yes you can add material same width as the fore end to make the fore end deeper up to a max of 120 mm from the bore line. easiest way is some nice wood, and make some tapped locations that fit the fore end rail for the mounting screws to secure into.

Or you could improve your position and use a better hold than fingertips.

Have Fun
Robin