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The model train industry has a product that spreads like drywall spackling but is light, around the same density and feel of foam board. It's usually used to make foam mountains smoother and hide seams. It's also non-shrinking and sandable after you've worked it into it's spot and let it cure. If you hit a hobby shop that carries train supplies, I bet they'd know what it's called. Probably not cheap, but good for the little gaps.
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