14 and the plans too hard to follow? No problem.
Go to this page:
http://www.gbfans.com/equipment/reference/
At the top you’ll find proton pack screen grabs. At the bottom you’ll find photos of the pack from plant hollywood. Print a few of those and use them as a reference.
Now check your dad’s garages for parts you can use. Grab everyhting you can find.
You’ll need stuff like the following:
Wires, hoses, and rubber and plastic tubes
Electric tape and duct tape
Small caps and knobs
Metal rods, wooden dowels
Screws and bolts
Large and small washers
Cans
Electrical boxes (the kind you mount light switches and plugs in when you put them in the wall)
Wood
Large detergent caps
Electric tape container
Knobs from stereos or radios
Red and blue clear plastic from folders or shampoo bottles
Flashlight or other battery powered lights preferably which flash.
Blue or red glow stick
Cake pan or other large round thing about 9" around for the cyclotron.
Bascially grab all the junk you can and throw it in a big pile.
Here’s a pack I built last year largely out of the junk I listed above:


The big ribbed hose is from a vaccum cleaner. The ribbed boxes are electrical boxes. The big box on the top left is a speaker box. You’ll need a big flat piece of wood to mount it to. Or you can use a big piece of carboard. If you’re gonna use a backpack find some way to connect that to this “motherboard” first.
Then just tape everything on there. That’s what I did. Put the tape on the inside if you can.
I cheated a bit in that the bottom half is all built from wood but if you just layer some wood up you can get something similar. You’ll have to glue it together though, tape won’t hold it. Or you could nail it. Keep in mind it’ll be heavy though. So you’ll want to keep the amount of wood you use low. My pack weighs about half as much as you do. 
If you use FOAM anywhere, you MUST cover it with tape or a lot of glue before you paint it. The paint will dissolve the foam. Unless you use latex paint. I’m talking about spraypaint.
Oh and the cylinder thing on the top right is a dtergent cap like I mentioned. And the cylynder on the bottom right on the cyclotron is a pringles can.
The bumper on the cyclotron is a rubber sheath for wiring with a coathanger inside to keep it straight. You might have a hard time finding that sort of thing though. You can make that out of cardboard instead.
The proton gun here is just a star wars blaster I removed all the extra parts from and painted black, and then drilled a hole in the front to stick in a piece of clear tubing.
For the hose leading to the gun you could do what I did initially, if you can’t find a black hose, and just take any old hose and cover it in black tape or paint it black (again with latex paint, spraypaint will likely melt a rubber hose).
Just be creative. As long as you have a bunch of boxes and wires on there and something that looks like a cyclotron nobody will know the difference!
Oh and don’t forget to print the labels up and glue them on there.
[edit]
Yeah if you have back problems you definitely don’t want any wood on there. You’ll need to use cardboard boxes or something. If there’s a post office near you, go in there and grab some of their priority mail boxes. They’re free, and square.
FedEx would have some too for free of smaller sizes. Or have your dad grab some.