I bought some ghostbusters patches of Ebay and a brown/khaki boiler suit.
I went to a local sports shop and bought some small knee pads for the elbows.
The back pack was a large cardboard box with an old ruck sack in it for the shoulder straps.
I went on Google and lots of forums looking at proton packs to get an idea of the layout.
I put some holes in it for lights to shine through and used a broken dyson for the gun/wand part fixed to the base of the box.
I also put a small USB pen drive style MP3 player in the wand and had ghostbusters sound efects (proton pack start, stop, Ecto-1 siren) and the ghostbusters theme tune on it and some speakers in the base of the back pack.
I ran a 3.5mm jack lead extension cable down the wand and into the speakers.
I added some cables (from the broken Dyson and some old ribbon cable).
I sprayed the back pack in black spray paint and that was that.
This is my third year posting my costumes on here and I am looking forward to seeing some of the other entries.
Good luck to everyone and carry on using and posting to this great website.