I’m not 100% sure, but I think it was around 25kg/m2 EPS so slightly less than 1.5pcf, the insert is XPS, I think around 2pcf…at nearly equal density, XPS feels much stiffer than EPS.
I used to use 1pcf as it was the only big blocks I could find before and never had any issues…so 1.5 should be plenty
For the wing board I made recently ( Where to buy foil track boxes? - #7 by Mat ) I used 2 “planks” of balsa instead of a block and it seems to work really well so far.
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