IMO it really needs to be a box. You can come up with whatever exact dimensions you want, but anywhere around the top of the extension to around the head tube, width of ~15cm, height from head tube to top of extensions.
You’d have to word it correctly and think about loopholes, but it’s a much cleaner solution than anything open to interpretation.
Of course the interpretation is always favored by governing bodies, clever solutions that are ugly can then be deemed illegal because reasons.