That would work.. And you could even make that from flat iron to make it stronger, just need to bend it twice and drill holes, that way you could make 2 holes in both ends and avoid the speaker from getting in wrong angle. That way you could also make it in specific angle you want the speaker to be in.
Edit: After looking at this I got an simple idea that would work and look decent as well.
You will need flat iron 1" wide for example cut in desired length and bend it to angle, drill 1 hole to fit threaded rod / bolt that fits your speaker thread, apply nut(s) to hold it on place and drill 2 holes for wall mounting. After you have finished it paint it to match your wall color and mount it.