Not a simple problem as you can literally have 100's of thousands of packing combinations from which to try to select an optimum configuration.
Are you trying to pack a shipping container efficiently from a list of boxes of different sizes/shapes to be shipped?
Are you tyring to pack different boxes from a list of smaller items to be shipped?
Or something else?
The latter - pack 4 standard shipping boxes with a variety of smaller boxes of about 12 different sizes. Not too many but still a bunch of combination possibilities.
Don't have anything much to suggest on that one other than to agree that the possible combinations are quite large.
Do the smaller boxes vary in height as well as length and width? (That makes this a possible 3D fit problem where "all the same height" or "only one layer of smaller boxes will fit" is a 2D fit problem--much less complex in terms of the possible "fit" combinations.