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I'm thinking about using 1/2" Duraboard as an elevated shelf to enable firing 2 shelves-worth of glass in one go. I would set one 12" Duraboard base on the floor of the kiln, then a second 12" Duraboard shelf would be elevated by 4" tall kiln posts.

I'd use kiln posts at the four corners and a Duraboard support in the center. (Eight 2"x3" triangles of Duraboard stacked to the height of the kiln posts.)

I fire small & light - most items are <2" square, and about 1/3 to 1/2 of my fires are single-layer tack fuses. But I'm also loading shelves pretty full these days in holiday production mode.

I fire in a 16" fiber kiln and use Duraboard (w/ fiber paper over) as shelves. They're not rigidized, but I've read a lot of negatives and hassles here related to rigidizing, so I'm skeptical of doing so.

My chief concern is that the Duraboard will weaken with repeated firings and eventually collapse into the glass on the kiln floor. True?

Looking for sage advice, heads-ups, flags, alternative solutions, and constructive suggestions!

Your board is probably small enough to be ok, but you may find that you're not happy with the results of the firing. The temperature at the top of the kiln will most likely be much much different than the temperature at the bottom of the kiln.

You'll have to experiment to see how much different things on the top shelf look than things on the bottom shelf.

Good point, Brad

FYI..I routinely fire 2 levels in my kiln I fire to 1385 to tack fuse on the shelf, and use the floor under the shelf to slump, this seems to work well, but there is a good bit of difference in the temp.

the other problem with that is that duraboard has more chance to drop flakes than a ceramic shelf above another