Raised hearth extension framing

[Blackhead]

I want to build a a hearth extension for my new Pacific Energy FP 30 zero clearance fireplace.
It must be constructed from non combustible material, including framing.
Should is use steel studs? concrete block?
For the base, I have a maximum height of 9-3/8”.
Any suggestions?

 

[begreen]

Will the hearth extension be right under the door? If so, yes. The hearth foundation can be made of cement block, or metal studs. The metal studs construction will weigh less. If used, they should be the heavy-duty variety.