Hearth Extension Desa B42LI-M

[ulcuss]

I bought a Desa Wood burning firebox 42” and start thinking about a hearth extension. Desa offers one HE42 ($106) 66"L X 20"D X 1"H but not much is known about material they using.
Desa's requirement for Heart Extension is K=0.84 BTU IN/FT, HR F at 1".

In my real world the thickness may not more than 1" plus tile layer.
Can someone tell me what alternatives I may use? (Durock, HardieBacker, etc)

Thanks,
Dieter