I actually went as a Ghostbuster over 20+ years ago right when the film first came out. I made by own backpack too.
I just used a regular backpack placed inside a cardboard box so only the straps would show. I cut slips for the straps. Used duct tape to seal all the edges, used wrapping paper tubes, black spray paint, an old speedometer (I opened it up and placed a cut out piece of paper with green to red paper inside so the needle looked like it could be powering up) a throttle cable hooked up to an old Han Solo Star Wars blaster. and 2 frisbees taped together with a reflector in the center hot glued to the back where you notice the discs in the movie. I used reflective (thick) pin stripping around one of the tubes to show "caution" and used a store bought sticker that said, "keep hands clear" and placed it on the backpack.
It took just 1 day to have this up and going and I was a hit! In fact I kept it for a long time after that and was telling someone the other day I went as a Ghostbuster complete with a homemade pack. It was really, really cool. You can go to the UPS store to find a suitable cardboard box for under $5.00.
