I think I got it, it was actually fairly simple, I still need to test it more, to make sure there are no hidden issues

add following for any shield in SYSTEMS.lua (XX is amount of damage per weapon fired at it, shield should absorb)
and you need this file

vanilla shields, Harpies have 2 hardpoints with 2 spiters each doing 15 damage each, i.e. 2 x 30 damage

shield absorbtion set to 10, each spitter does 15-10=5 damage only, 4x5=20 damage, 10 and 10 on the picture

shield absorbtion set to 15, each spitter does 0 damage