After a liitle research, I like the fiberglass idea, but my main concern would be cost of the overall volume of the supplies I'd need to recreate the majority of the wood in the habitat. It's a big habitat and lots of branches & logs collected over the years (though not all will need replacing).
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One idea I found that I kinda liked would work with f/glass or a concoction of concrete or plaster. It uses pool noodles as forms with bent PVC pipe inside the noodle for strength. See pic for an example. They carved/sculpted the pool noodles into more organic shapes and you could join pieces if you wanted branches on the branches or branched shapes -- they used Great Stuff as an adhesive. This guide was for making indoor props, so they did a sort of paper mâché type application with cheese cloth and glue to cover the noodle and it took on the shape they'd sculpted into the foam. But fiberglass would work. Or maybe a mortar mix might work -- would take some experimentation.
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