I don't have many at the moment and am thinking of just storing the Eurolite cases in a closet. The bookcase would take up less space than having multiple cases lying about.

https://www.ikea.com/us/en/catalog/products/80263832/

I have a load of these. The thing to bear in mind is that the shelves don't have a lot of tensile strength - a shelfful of LPs or LaserDiscs is much heavier than they're designed for. In practice, the 40cm-wide version seems to survive OK; the 80cm-wide version definitely won't without extra support. (I've broken the 80cm shelves just with books before...)

They're also not as deep as a 12" sleeve, so the discs will stick out the front.

Why not KALLAX?

I guess I could go with https://www.ikea.com/us/en/catalog/products/60275812/

though I didn't want something big like that horizontal or too tall

Ikea stuff is total garbage, will never survive a move without ending up looking like crap. The stuff is particle board with vinyl vaneer glued on. It peels.

The dimensions on Ikea stuff is always off for some reason as well.

If you don't have enough money for real furniture/timber or metal shelves I'd recommend Muji pulp board storage, cheaper, lighter and more durable than Ikea.

If you attach them to the wall and you don’t pack them KALLAX is pretty good. For LD on a 3x3 or more I’d plan to leave every other cube empty or to fill with something lighter. That or just leave every cube half empty.

I wouldn't waste the money on particle board. Try and find something better otherwise you will be buying more again in a year or two.
Or one day you come home and all your discs are splayed on the floor and you are hoping that you don't have any major damaged discs.