I have a contact in a business that uses a lot of 4 1/8" x 9 1/2" envelopes, and the 500 count boxes those come in are great for boards.

CPU are in various tubs with that grey antistatic foam lining them, not real organised.

Antistatic bags of relevant size, using a piece of foam for cpus with pins

Then they go into a cardboard box (laptop shipping boxes work great for mobos).

I bought a bunch of motherboard sized anti static bags and I store the boards in the bags in a plastic container I got at The Container Store which is just big enough for the boards to stand up on edge. I have a similar smaller box for video, sound, and other types of expansion cards. I won't store those components in a container which is not sealed and waterproof.

Motherboards in ESD bags and stored in motherboard boxes or in a few cases stored sideways in larger boxes with sheets of cardboard between each board (because I didn't have enough motherboard boxes for all my boards). Important here is that none can scratch each other when handled and that none will bend over time or otherwise gets damaged. Also all are stored without a battery, just to be on the safe side.
In some cases I'll store the motherboard boxes in kiwi boxes as these are sturdy and made to be stacked and costed me nothing to get.

CPUs in CPU trays or packaged in several different other ways in case I don't have fitting CPU trays (in little ESD bags 2 each with the pins together in some of these cases).

There have been other threads about storing computer parts btw. This subject comes up from time to time, for obvious reasons 😜

Some time ago I made some 3D printable CPU trays and RAM holders. I uploaded them to Thingiverse. There are remixes of my CPU trays for other cpu types available too. https://www.thingiverse.com/barreljack/designs