Ok gang, here's what I wound up doing...
On Wednesday afternoon I visited my local UPS Store and bought five boxes that were sized 24"x18"x6". Just a tad
deeper than what I originally wanted, but I can still stack all five boxes on a rolling platform under the layout. [And as I mentioned previously, these boxes are only intended for
storing my freight cars off-layout, NOT for transporting them - I will use the SMD boxes if that need arises.] On Thursday morning, I went to the nearest JoAnne Fabrics store and bought a 72 -inch long roll of 1" thick cushioning foam which I cut into sheets to line the bottoms of each box.
What I decided to do was try to put like-sized cars and types in their own boxes, rather than randomly as they previously had been. I focused on getting all of my 50ft-and-under length cars into the UPS boxes while saving the SMD boxes for the longer ones.
Here is a UPS box filled with 50ft boxcars, with a bit of leftover space for the only 4 propane tank cars I have in my roster:
Here is why I wanted boxes that were at least 24" long, so I could easily fit in up to 3 columns of cars at least 50ft long, plus couplers. Shorter cars, such as 3-bay coal hoppers, can fit 4 columns across.
The UPS boxes, even though they are deeper than desired, can still be stacked up to five high (although I only needed
four) and easily rolled under my benchwork:
With all of my shorter cars packed into the UPS boxes by size and type, I gained space in the SMD boxes for longer cars like my 86ft Hi-Cubes and 89ft auto racks:
Finally, to make each car easier to locate when I want to put them back on the layout, I exported my JMRI cars roster to an Excel spreadsheet, sorted them by road number, and added a column at the far left for filling-in the box numbers of where I stored them:
I numbered the UPS boxes in the "100" series to easily differentiate them from the SMD boxes.
P.S. As a result of optimizing the storage of my shorter freight cars, I wound up with three (3) leftover empty SMD boxes that I can use if I ever need to haul any trains to be operated on a modular club layout.