sandy loam or sand
lined/sealed deep empty pit with interchangeable covers
a. metal grate
b. metal plate (i.e. "diamond-plate")
c. wood planks
e. can also be used as a water pit, hence the "deep" descriptor (though you'd need some sort of pump to empty it)
I'd also keep some loose ...
Surface as amp is an often mentioned technique in texts on recording foley.
I myself don't like the sound of a keyboard on a hard surface, usually the surface resonates too much, but on the other hand, on anything soft a keyboard can sound really thin. That said, the project dictates the sound you're after..
Also, don't record avoid recording with a phone, ...