@OMW, nice to see your ideas, too - I ended up not using pink/blue styrofoam at all (although I will probably use small bits as statue pedestals).
Upon studying the screen captures and the Weta environment, I decided that the table is about 4 cm in diameter, and the inner part of the chamber is 20 cm across. The outer rim with columns is about 6 cm wide, making the outer diameter of the entire chamber 32 cm. For the height of the outer wall I chose 9 cm. Using screen caps as a help, I quickly designed the following templates:
The chamber floor plan (or section thereof):
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The chamber outer wall:
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The chamber inner wall; here my measurements seemed a bit off, and I ended up having taller wall above the doorways than what the movie shows. Consequently, I had to extend / extrapolate the elven ornaments a bit higher than they are in the movie chamber.
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My plan was to cut the wall surfaces from 3mm Depron, with an overall thickness of the walls being 12 mm – the openings have a tricky layered / beveled structure, which calls for four layers of 3mm Depron sandwiched together. The final template pic shows the walls facing inside the round column “corridor”.
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I’m building a section with three outer and three inner columns, i.e. two full arches on both sides. As a starting point, I cut a section of the outer ring ceiling / roof from 5mm foamboard. A 6mm wide strip of 5mm thick foamboard is glued to the underside. The purpose of these is mainly to be the attachment points to the inside-facing walls. All the columns seem to have half-round part on the side, and I decided to use suitable plaster castings from Hirst Arts molds for the column details. Here’s an overall pic of the parts laid out & templates cut to size.
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I started by gluing the foamboard strips to the roof part, followed by the attaching of the outer wall cut from 3mm Depron:
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Next, the inward-facing wall. Note that the columns are separate for the outer wall, so the Depron parts only extend as far down as the column tops.
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Small bits of 5mm foamboard were glued in between the layers to allow more secure attachment to the column tops later on.
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Next, the innermost wall – here, the columns are more integrated as part of the wall, so I cut the Depron part all the way down to ground level.
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Here, too, I used 5mm foamboard strips as supports between the front and backside of the inner wall.
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(note: edited the link to the "hidden" walls template pic; the measurements in the original pic were incorrect)