I assume the problem is that you cannot fly a drone inside your structure so that you could find a volume of the stockpile. This would then require you to do some terrestrial photogrammetry assuming you can walk around the stockpile. You might want to try using PIX4Dcatch with your mobile device. It might work but indoors areas can be very challenging. But it is worth a shot.
I had fixed wing drone from Ebee( Ebee Plus) but now I have bought a small drone DJI Mavic 2 Pro that manually I think it is possible to fly inside the shed that we have.
The issue is on the acquired photos. Is it possible to be exported from the drone and processed to get the 3D piles? Based on the articles I have read, the drone must be flown in off GPS mode, How are the taken photos be georerenced/ geotagged without GPS coordinates?
We don’t have many, if any, users who are flying indoors to map stockpiles. The drone might still be able to acquire a GPS signal, albeit not accurate, through the roof. I would try manually flying and see what happens when you bring it into Mapper.