Speaker enclosures on the porch

I wanted to have some speakers on the porch outside of the front door. I thought it would be nice for background music while looking out over the bowl. I had the brackets, but they weren't very stiff, so I thought to back it with some plywood.

Then I figured why not build a box around it as well, to get better bass response. I hope it works out, I am not too sure of the air tightness of the enclosure.

Also, I am a little worried, since the bottom board isn't attached to the frame on the sides. It is only attached to the bracket and the bracket to the joists...