Amazing modeling Pal !

Since the HDRI is locked, start your scene, toss the I-plane, create a 2-D elipse object(layout flat) and apply a mirror mat. Now you can determine HDRI direction. Rotate your scene to preference, and start building your scene once you've acheived a desired angle.

I did a few scenes in Carrara where you can rotate the HDRI mapping, so when I realised I couldn't do that in Bryce I figured why not Rotate the whole scene.

I like Dans Idea..(grin)
Now I'm inspired to attempt something similar in modeling clay.

Happy Holly-Daze

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Ockham's razor- It's that simple