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The recessed box can support a speaker (using the cut-out) and the keypad. I think there's a post here where someone added pictures. NOTE: speaker needs to be provided with sound source from the ELK; you need a separate set of speaker cables for this.
OK, thanks for the tip, I found it
here.
It also answers my other question; I simply need to remove the speaker from an SP12F and install it in the recessed box.
My list of parts is growing (3 sirens, 3 keypads, 1 strobe) so it looks like I will need an auxiliary power supply. What are the recommended solutions and Elk part numbers? I found the ELK-412K but Digger posted that he used something else.