I've been looking for storage units to put under my long arm for almost as long as I've owned it. I've had a mishmash of containers, plastic drawers, wine bottle holder and baskets.
I wanted something that offered protection from dust and lint, but that I could see into. Because I also lower the frame at times, it couldn't be tall storage containers. Something reasonably attractive as well.
The Sunday paper had Michael's Arts & Crafts sales ad and the
Jetmax Modular Storage units were on sale. I have one of these units in my sewing room and it has stood up to sliding the drawers in an out countless times, and the weight of thread and other items reasonably well. Not much wear at all.
The units are basically 14.25" inch cubes with a variety of options, drawer units, shelves available that you put together with with a phillips head screwdriver and they have good assembly directions. I chose these four units. Three with drawers with clear fronts, and one open cube to store pantographs and my cloth side extenders. I also purchased the pegboard kit with hooks that is available for three sides of two of the cubes. All my Circle Lord stepping discs fit in one of the drawers. The drawers are tall enough to store cones of thread upright.
This picture is of the open cube on wheels for the pantographs.

Here are two of the units under one side of the frame. No more wasted space under the frame, and some nice storage.