The Container Store has some boxes that take G scale stock - I just got one which handles 4/5 freights.
In the background [inside] you can see a flatcar and a towel. I use old towels wrapped around my lighter objects to protect the details.
You’ll notice the box is 60Q/56.8L. Most of the boxes are sized by volume, so you just have to go and look. Other sources include Target and Office Depot.
An irritating feature is that, as they stack, they taper towards the bottom, restricting the size. Measure the bottom of the box with a tape measure, and ignore the dimensions on the label.
I do too, and for my electric locos, but they are not very long, so it only works with the short locos. Again, I wrap them in old towels and pad them with bits of foam. (Towels can take a bit of heat - I often drive home with the top of the box open.)
For most tender engines it is difficult to find a plastic box long enough and tall enough, without it having an enormous waste of space inside! Aristo heavyweight coaches (and USAT streamliners) are another problem - I keep them and carry them in the original boxes.