As for the height there is a 24" minimum from combustable surfaces with proper ventillation provided. The sides require at least 12" from combustable surfaces.
The unit can not be fully enclosed on the sides. Maximum from the back of the unit is one third the surface of the unit. The rest must remain open for at least 24" on either side of the unit.
So if you have a 30" counter top with a typical slide in unit you can have a mantle at 24" above the unit with 10" supports on the sides measuring from the wall out. The supports must be 12" from the stove edge.
Now if you were to cover the supports with a cementious material and show that it is a fire resistant surface then you may be allowed to bring them in closer. But that will take some serious convincing of the inspector to do that.
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