Hi Working on it, good to know, I also remember this spreadsheet by Andrew Gibson.
I mentioned the 3D model since a certain CAD modeling software could do an instant weight calculation, while the spreadsheet involves more effort to enter data and calculate material volumes after making changes in a CAD model. The idea here is to design in CAD and reach the weight goal during design phase, prior to committing the build start. As far as I could find out the Fusion 360 free non-commercial version is limited in features and I could not find how to change materials and see volumes.
I am also looking into ultra-light camper design and into free CAD software that could show material volumes to assess the build weight during design phase.
Cheers!