In fact seems a Scale 2D Front then 2D Front Right gives a cool result!

Separate the volume so all faces are free
Show Points
Select only all Points on a 3D side (so the principal face don't be changed)
Make a Front Scale 2D from 3D middle center
Make the same value Right Scale 2D from the same 3D middle center
Join all

You have a tapered volume without Box Defomation!
I will com back with the good second scale ;)
because here there is a little error!