The Rambo wool cooler is great for this IMO.
I layer but sometimes I think the combined weight and pull can be uncomfortable long term for horses. Like how a badly fitting bra/sweater can be uncomfortable for us.
Just adding a cooler as the bottom layer helps significantly. I would guess the cheapie fleece coolers add about 50g worth of warmth. The wools probably add 150g. My unscientific observation after running my hands under their blanket, of course.