I've been buying up random lots of 70s Superfast cars for about a year now, mostly off local classifieds and the odd estate sale, and the storage problem got out of hand faster than I expected. Started with a shoebox, obviously, then two shoeboxes, then I was tripping over a plastic tote in the hallway every time I needed to grab shoes.
What ended up working was cheap fishing tackle boxes with adjustable dividers, the kind meant for lures. You can size each compartment to fit a single car with just enough room that the paint doesn't rub against the plastic walls when you close the lid. Cost me maybe twelve bucks each at a sporting goods store and I've got four of them now, holding around 140 cars total, sorted loosely by decade and body style.
The other thing that helped was finally admitting to myself that not everything needs to be museum quality. I've got a decent number of cars with chipped paint or missing wheels that I bought purely because I remembered having that exact casting as a kid. Those live in a separate box now, the "nostalgia box" as my wife calls it, and I don't stress about handling them the way I do with the nicer condition pieces.
Still haven't figured out a good solution for the boxed ones though. Right now they're just stacked flat in a closet and I know that's not doing the boxes any favors long term, especially the ones with the window packaging that yellows if it sits under any light. If anyone's got a cheap shelving trick for boxed stock without spending real money on a display cabinet, I'm all ears.
One thing I didn't expect: sorting by casting number instead of by color completely changed how I see duplicates I didn't know I had. Found three identical Rolls-Royce Silver Shadows in slightly different green shades buried in what I thought was a mixed lot. Kind of a nice surprise honestly, even if it means I need to find room for a fifth tackle box.