An ipac 2 should be more than enough if you are using two u360s...the only issue you may run into is you won't be able to easily swap out you main sticks if you don't like them. I did the same thing the first time I built a 4 player panel, but I then realized that I liked the JLWs a lot better than the u360 sticks. I eventually removed the u360s, moved the JLWs to players 1 and 2 and installed two x arcade sticks I had lying around. I was using a keywiz for my encoder, which was not quite enough inputs, so I hacked a gamepad for the extra few inputs I needed. I later sold that cabinet. When I built my last 4 player panel, I used four JLWs and an ipac 4...I'm very happy with it. My u360s ended up in my 2 player cabinet that doesn't get used as much.
As for mounting, I like to under mount them using t-nuts, I recessed about half the thickness of the panel with a router to keep the height good.
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