I use plastic containers with lids. Large plastic containers that is.
If you have a Big Lots near you, they have some Stirling (I think)
under bed containers with lid. They are about 15" x 30" x 8". I think
they were about 8-bucks.

For some of the larger stuff like grinding wheels and buffs, I make
dividers with strips of ply wood that interlock like cardboard
separators in a case of wine.

I also use square food containers without lids and fill the under bed
tray so they stay pretty much in place. The large under bed thing sits
on a shelf and I only have to open a single lid to see what's inside.
I label the under bed thing with "Wood/Metal screws", "SAE Bolts/
nuts" etc.

Often HF also has specials (less than 20c per) on those rear hang
parts bins and I get a bunch of those and put them inside the under
bed thing.

I also have two carriers for the rear hang parts bin. I got a 4' x 4'
sheet of 3/4" particle board, mounted to a base with some casters and
side supports then stapled the parts bin racks to the 4' x 4'. I have
a curtain made from a table cloth from a swap meet (80c) that I drop
over the lot to keep them dust free.

Dave