So the reason you probably havent gotten the most detailed replies about this is that its not really a simple answer. The spirit pack basically comes with a "motherboard" already attached; I believe it is a flat piece of plastic that forms the back, but Im not sure. Normally you would take the shell of the proton pack that you are building and attach it with L brackets at specific points along the side of the shell (two points under the cyclotron, one at the ion arm and one at the gun mount). These points should be pretty well documented on the forums and you should be able to find more information about it by doing a search of the forums.
However, the spirit pack is a very different animal from the "standard" proton pack kit. It is meant to be a complete prop, not as a shell to be added to a motherboard. That means you will have to do some modding.
Your first step should be to see how to attach a motherboard to a proton pack shell (take a look at the MANY build threads on the forum and eventually you will see how it is done in detail). Once you have a good idea how this is done (Maybe you already do, I dont know your life), try to do some digging on how to remove the backing on the spirit pack, or find some teardown instructions. Get a good idea of what the spirit pack looks like inside:
This thread has some good pictures of the internals about halfway down
viewtopic.php?t=43678 Now you just apply the process for putting a motherboard from a full size pack to making a motherboard for and attaching it to a spirit pack.
If your end goal is to attach the spirit pack to an Alice pack frame, some people have just drilled some holes to the pack and zip tied it down, its not super "accurate", but it will get the point across.
You may also be able to attach an alice frame directly to the material covering the back of the pack. Keep in mind that I have never seen a spirit pack from closer than 10ft away and I have 0% of an idea what it is made of, so I have no clue as to how strong it is, but it MAAAAAYYYYY work.
So there you go; something to at least give you a direction. Also, I only sporadically check these boards, so if you do have a question for me 1. I will likely not see it and 2. I wont be much help anyway because, again, I have never seen a spirit pack from closer than 10ft away.
Good Luck, Have Fun, Dont Die