Well, I got this all setup and in a project box, with four 1/8" female jacks and the power jack. I also had to drill an additional hole for the IR sensor, and it works great!
The only challenge I had is the programming of the Harmony Ultimate. The IR relay board came with a remote I used to teach the IR codes to the Harmony, but it doesn't have discrete on/off IR codes. So it uses the same code to turn trigger 1 on and off; as well as for the other 3 relays. But, the remote also has a "power all" button. This button turns all 4 triggers on if none are currently on, or turns all 4 off if 1 or more are already on.
So for programming the Harmony, in order to get the DIY trigger controller into a known state, I have the harmony send a trigger 4 command, then a power all command. This powers all 4 channels off, and gets me into a known state. It works for me because trigger 4 will never be the only trigger powered on in any scenario. If is was the only trigger powered on, then sending a trigger 4 command and a power all command, would turn all 4 triggers on.
So I now have my system setup to when an activity is chosen, the amps stay off and everything else powers up. Then I have a macro (or sequence as Harmony calls it) assigned to a button on the remote which goes into the TV's menu and toggles the TV speakers on/off. So I've trained the kids, and my wife, to use this macro button if they don't hear any sound. Then when I want the full system, I press the macro button to toggle the TV speakers off, and then I've mapped a button on the Harmony to turn trigger one on (which feeds a trigger controlled power strip to which the amps are plugged in to).
And the "off" command of the harmony always sends the power off sequence to the trigger box.
For $35, this is a very sensible solution for an IR controlled 12v trigger controller.