Solid Frustum Unwrap using UnwrapCone2 script and Flow, and other routine MoI commands.
("Final" summary of procedure.)

Begin with a solid Frustum, which is a cone with a smaller cone located inside, with the same axis, with the tops cut off with a plane. Thickness is constant. (e.g. Sitting in Top or Bottom view) (Join may be used to create the solid.)

Use the script UnwrapCone2 to unwrap the outer cone surface, to produce 3 arcs and two lines. Trim off the apex portion of the unwrap, to leave the planar outline of the frustum. Trim away half of the planar arc.
Sweep the planar (half) frustum with one edge line, to make curvy isocurves, visible with Curve>Iso command. This planar arc surface is the Target for Flow. (Rectangular isocurves do not work here. The planar isocurves must correspond to the isocurves on the outer frustum surface.)

(e.g. in Front View) Create a plane passing through the slantseam, to divide the frustum into two portions. Return to Top view and Discard half of the frustum. The make the remaining half a solid, may need another trim utilizing the plane, and a join.)

Copy the remaining (half) outer face of the solid (half) 3D frustum, off to the side, and set it to a different color Style, to facilitate hiding and unhiding selection in Flow to come. This will be the Base surface for Flow.

Select the 3D (half) frustum solid, and select Flow. Follow the Flow directions.
Hide the solid using its color Style eye, and select the previously prepared Base surface, near a corner. Select the target planar arc near corresponding corner, and complete the Flow.
The result is thick, flat, unwrap of half of the solid frustum, with the arc sides beveled. This can easily be doubled.
(Minor face artifacts could be deleted and replaced with ending sweep???)


-Brian

Flow of Full frustum was not successful...
I imagine that a hole in the half frustum solid would also transfer with flow???