It's a ritual, they are always at least 20th level, even if they are less than that in the determination of magnitude.
Look at this spell for inspiration, and run the final spell designed through the troupe for some comments...
That spell creates various pieces of wood of differing sizes; this spell would need complexity to assemble all the pieces into a final form. I think T: Group and Base 2 are better than T:Ind Base 3 (which is like a huge hollowed out hunk of wood of gargantuan size. But either approach is possible. I just want someone else's eyeballs on the final spell designed/submitted.
Or...go hire a specialist. 