Suggest to having ability for different area when designing on area page, customer can select which part to design, step to step.
For example , there is 2 pocket on T-Shirt. To choice design 1 by 1. (having bounding box may be), so the final design work will look better then using mask . Mask will hide the design, sometime is hard to view. or may be Mask can be setting different transparency , so customer can locate what they are designing.
hope this make sense , just suggest
Can choice which to design , and design each by each