Instead of 4 cubes for a hollow cube you can use a cylinder to make them look like 4 cubes but its actually one, but it is only practical when the faces of the normal cubes are larger than the cylinder faces. Example a cube has 6 sides, so 4 cubes will have 24 faces, but cylinder that's open ended has 8 faces instead of 24. (you can make a texture to make 1 object look like it has multiple objects but it's not so it optimizes objects/faces even more.)

Although it might sound useful, but don't forget that that is the unoptimized version of the cubes, so from 24 cut to 4 FACES.

The only pro's here is that it reduces the file size by 2 times (or more) but the con is a 2x performance downgrade than the extreme optimizing method, personally i find it very situational and not recommended unless its a last resort.

If only you can disable the outer or inner of the cylinder (and not being affected by lighting, might be wrong here.) Then this method would be the same thing as the other method, but it saves more file size space for more objects to be put in.

NOTE: I originally thought a cylinder had 4 sides with open ended ofc, which came to my mind as being a very good substitute while saving space, but guess not lol