As far as I understood, you want to create a 2D print and thermoform the print into 3D. Therefore you need a reverse transformation and this can be approximated with Kangaroo. According to your photo above, the plastic sheet is pulled into a negative cavity and the points which are on the flat plane do not move. Hence I fixed all points which lie on the XY plane. They don’t move in the Kangaroo simulation. All other points are free and are pulled into your 3D shape.
You will have to model your print on the 3D model. Mesh Cage Morph can be used to transform the control points of the print outlines from a 3D mesh to a 2D flat mesh.
You’ll need the fattener.gha addition which you can download here.
thermo_transform.gh (146.6 KB)