How to implement light armor?

So let's say a character wants to buy half plate that is 80% of normal weight. How is this reflected in the armor mods?

I am not sure weight does anything with Armor Penalties.

In the past I've always seen something like this for special armor:

Chain Armor (AT14) protects as AT 20

Or something like that.

Chain that protects as Plate on the left, regular Plate on the right:
https://i.imgur.com/I5jkyMX.png

Sulimo’s version above is the usual for many of Rolemaster’s Magic Armours.

The alternative is just having the weight be reduced with no other armour mods.
So just edit the Item in Inventory to weigh less but keep all of its other modifiers besides maybe Minimum and Maximum Maneuver Penalties(Qu/Missile penalties are more due to restricted movement from form-factor/pauldrons/greaves vs due to weight).
Done.

I would also change the MM skill to Chain instead of Plate.