MegaEdit has a number of default scripts avaliable to use (MegaEdit PRO version ONLY).  Here we look at the Mask support script and how to apply to a product.

Firstly enable the script against your chosen dynamic product under the dynamic products setup displayed below.

You are now required to set a tag of "mask" against a field under the options tab of a fields pop up which will activate the script and that chosen field will remain the top layer when new text or image fields are added.

Below I have added a single image field and that is automatically placed behind my mask tagged field.

Then below I have added a second image field and that is placed behind my mask field, but, atop of the previous image field.

All subsequent fields are placed behind the mask layer.