Given a design which has one instance of a 3D element:
One would replicate this by:
determining a minimum stock size on which the 3D model could be exported from:
first draw a rectangle using ctrl (command on a Mac) corner-to-corner clicking which has its origin at the lower left corner:
copy-paste the geometry which describes the element in question:
Boolean union the two:
OK
mirror horizontally:
and vertically:
and then align with the original geometry:
and draw a rectangle corner-to-corner to get the stock size:
Ok
Export the 3D model as a PNG:
Draw in some geometry to check the thickness of the 3D model:
Revert back to the original file:
Draw in a rectangle which is the size of the previous stock:
Align it with one of the elements which needs the 3D:
Using the rectangle:
Import Image:
Invert
Repeat for each element.