This is more topologically complex than you are likely thinking it is, particularly if you want to use material that isn’t ~infinitely thin. Because of the angles, any finger joints will have surfaces meeting at different angles on different planes. Without making angled cuts (which would be difficult to do on your Glowforge), finger joints are not going to mesh neatly. There are going to be gaps (which you can likely fill with glue). Any vector design is going to be very fitted to your material thickness.
Consider starting with a paper model to get a feel for the issues at the ~infinitely thin state. Then move to cardboard.
You might be able to model something workable in a 3D modeling program by lining the sides up and using Boolean operations to cut finger joints. Again, it will need to match up to your specific material thickness pretty well.
An alternative approach might be to build your box in cross section slices (i.e., a stack of squares with each one being slightly smaller than the previous). The sides would be a little stair-stepped but, you could sand them flat.
Like this:
Someone else may have better insight or better suggestions for this.