That's my friend's name for it, and I like it.

I see this project as having 2 parts:

1. A harness-like garment that can be worn over or under clothes to house the artificial breast and allow access to storage. This is the 3-point adaptive pouch part. I say "3-point" because it is held to the body with 3 straps. I really need a picture to explain what I'm thinking.

2. A breast-shaped prosthetic/form with an accessible, hollow interior that allows storage of personal items.

I need to start with the storage prosthetic, then I'll have something to build the harness around.

So I did. In this case, I want to 3d print the prosthetic in a flexible material, like TPU. Conventionally, artificial breasts are cast out of silicone. This creates a body with a weight and flexibility similar to a natural breast, but natural breasts are not considered practical storage devices.

I started by finding existing "forms" that I can 3D print, explicitly permitting remixing.

I've chosen Takposha's "Breast Forms" [Printables] model as my starting point.

Methodology

My "plan" is to create a boob-independent tool-set, that will allow me to create a "utilititty" out of any compatible breast-shaped STEP file.

1. Create Access Ports

Shaped cuts in the hollow form are designed to work with the existing structure of the form to preserve shape, structural integrity and surface area, while providing a flexible, valve-like opening to the storage compartment. The rounded shaping of the form makes this opening easy to pass items in, but requires more deliberate force to pull items out.

Fun Fact! 
Fusion 360 becomes unbearably slow after only a few hours of working on a model. I can remediate this by disabling history, but it seems silly that it works this way at all. I am at the point where it takes a literal 5 minutes to press/pull the face of a simple polygon. It's so bad, I assume I must be using Fusion360 wrong? If someone can explain how I can avoid this, I would be much obliged.

2. Hollow (or Thicken) the Form

This was a lot harder than it had any right to be. By default, Takposha's Breast Form is provided as a 1mm thick mesh file. I can convert it to a solid object easily enough, but despite being designed as printable without supports, that doesn't work well with TPU (a flexible printer filament).

at 1mm thickness, there isn't enough surface area to support the peak of the form in TPU.

Appropriate techniques for increasing the thickness of a (relatively) simple solid include the following:

• Fusion 360 Shell Tool - Attempting to "shell" the form causes Fusion 360 to painfully crash over the course of 5 minutes. This is likely because my solid is a converted mesh with 1000+ triangles, and not easily defined mathematically.
• Fusion 360 Thicken Surface Tool - Generating a mutable surface based upon constraints derived from a source sketch or solid. This also causes a painful crash, for the same reasons I was unable to shell it.
• Fusion 360 Cut Tool -  This is not a graceful solution. This is brute-forcing my desired shape. It was my last resort, and it took me 10 minutes. 

  Crashing Fusion 360 because "too much math" is a slow process. I spent about 3 hours trying to learn the "right way" to adjust the thickness of my form.
  After a night of sleep, I gave it one more hour, learned how to convert my solid to a surface, and I made it work.

 If I were clever, I would create my own breast form with math, but that's not the problem my brain wants to solve right now.

3. Apply "Back" Surface

The base, bottom, or back of the form is important for this model because it's intended to safely store stuff. Without a back of any sort, users would get poked in the chest by their keys. I cut narrow slits across the back to give it flexibility without compromising the structure. I placed a hole under the "peak" of the form to help make support removal easier.

4. Test Print

Scale 70%. I can size and shape more carefully after I confirm that the basic design is feasible.

Filament: TPU

Print Settings: .3mm layers, Draft Quality

Custom Supports on overhangs < 30 degrees

Minimum model thickness 2mm, extra perimeters as needed.