I don't know if you can ever achieve screen-accuracy, but I had a lot of luck just using dollar store and found objects for my daughter's proton pack two years ago. You're a MASTER of using found objects and since this thread is about Ghostbusters on the cheap I thought you might be interested in a couple pics of what I did for her pack.
NOTE: This is scaled for an 8yr old.
Some of the items I remember would be the two halves of the toilet role holder, an electrical box, two flutes (orange and green), the pink handle of a fairy wand and lots of bottle caps.
Again some of the items here are kazues (orange and yellow), another flute (cut short), a water bottle top... and a few more pen caps and bottle tops.
This is another part of the pack. I used another electrical box, poker chips, two pencils, a check box, a pen and a tic-tac container.
More items... two sections of PVC, a small card board box, a purple soap travel holder and a juice bottle cap. The green box and wires were a small battery powered set of LED Christmas lights I used to light up the row of blue power lights on the pack.
Here's shot of the main section of the pack with the wand set nearby (all ready to paint). The wand was made of some wood scraps, a dowel rod, foam board, and other random bits and pieces.
OK, so here's the post-painting version with some old computer wires I had laying around and a bit of rub-n-buff to create that weathered or scuffed paint look...
And here is the same pack with the LED lights on...
And here is a look at the goggles I whipped up to go with the costume...
It was a lot of fun to build and very, very cheap. :thumbsup
Good luck with your build - I'm sure yours will be 100% cooler! I can't wait to see what you create!!!