There's some pics of how I've stored my stuff in this thread Re: post up pics of your "computing area"
My attic doesn't look exactly like this anymore, but in short I went 3D and got me some of those storage closets dunnotheirenglishname and lots of kiwi boxes 😁
I also got heaps of ESD bags and I would recommend you do the same. Try to prevent stacking PCBs too high or don't stack them at all if avoidable.
To a certain degree you can stack PCBs (like of course memory modules and small cards like older PCI VGA cards like Virges (individually packed in an ESD bag of course)).
Try to not store motherboards with their batteries installed or newer boards (like Pentium 4 and A64) with their often large heatsink mounted as over time this can warp the board. I use motherboard boxes to store them, they are just perfect for them. You can use sheets of cardboard to prevent PCBs from scratching into eachother when you have to move a box or something and this is very cheap as well.
I've also used old shoe boxes for stuff like Slot 1 CPUs, but these boxes themselves are not very stackable.
But your main issues will be to prevent scratching and warping of PCBs while also preventing damage by ESD and by accidents (like don't store a box full of CPUs on the edge of a table etc).
Once I'm done with my attic I can make some pics and share them to give some ideas. Particularly things like for instance CPUs can be a challenge to store properly and efficiently if you don't have stacks of CPU trays available.