For the inventory, a limited number of slots per inventory plus a weight limit. Same for containers, and if you have a container in your inventory, take the total weight of all items in the container, multiply it by the container's weight factor, and then add the weight of the container itself to define its weight.
Say you have (for instance) a duffel bag, weighs 1kg on its own, with a factor of 0.6 and 8 slots. You're storing a few rifles in it, each one weighing about 2.4kg. Say there's two of them, plus eleven clips weighing in at 0.8kg each.
As well as this duffel bag you're carrying, in your own base inventory, another rifle of the same model (you have the spares for parts), two clips for it, and two grenades at 0.5kg apiece.
So that's four items in your inventory (the clips and grenades stack), weighing in at:
2.4kg
0.8 * 2 = 1.6kg
0.5 * 2 = 1kg
(2.4 * 2 = 4.8) + (0.8 * 11 = 8.8) * 0.6 = 8.16kg
So the total weight is 2.4 (rifle) + 1.6 (clips) + 1 (grenades) + 8.16 (bag of stuff) kilograms, 13.16 kilograms. The gear in the bag, if carried alone, weighs in at 13.6 which is more than the total.
Of course it's more convenient to carry everything you're not actively using in containers, which would be encouraged by limiting the number of inventory slots the player has to something like 8, then totalling their strength for encumbrance.
And like FO3, if you're overencumbered, change to a slower datablock instead of stopping you from picking things up. If you're over your slot count or your weight limit, you're overencumbered.
oh god big post and complicated idea
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