Mounting A Bat House to a 4 x 4 Post

I've searched the sub, and looked at the guide the bot replies with. Neither helped.

I want to attach a fairly large bat house to the SIDE of a 4x4 square wooden post in my garden. Unfortunately, I'm not sure what hardware I would need. I see plenty of options for attaching to the TOP of the post, but not the side. Does anyone have any suggestions, especailly ones that aren't expensive?

Less important question. Would it be "okay" to attach a second, small house immediately below the big one?