I wired a friend's panel using the Minipac and their wiring harness. Everything was super easy. The only wiring harness issue I had was that I had to extend some of the wires to reach some of the farther buttons (the harness extends to a max of 24").
Two Minipac issues: One, it comes with a USB cable, implying that one should connect it to a USB port. But its jumper which is used to switch between PS2 and USB comes preset to PS2. I didn't find any docs which documented this. Luckily I had access to this group and got this resolved quickly

Two, I couldn't program a different set of inputs. I switched the jumper to the ALT setting, but I couldn't get a newly defined set to be programmed. I thought I was doing everything correctly, and read all the docs about it. But I just gave up and ended up changing the MAME frontend instead (which was easy, so it wasn't a big deal).
Everything worked great when done. We used a Happ trackball and that works very well.
Mario