As heavyweight as possible, together with a hood. Does the horse come in at night? If so you could also use leg wraps to help keep him or her warm - apparently they have the same effect as putting another rug on.
Yeah she does come in at night - didnt know that about the leg wraps! may give it a try.
I have got a Rambo duo on her 300g liner with a hood, which i thought would keep her toasty but she doesnt feel overly warm at the mo so hoping i dont need to get something heavier in proper winter! Do you even get much heaver than that?
On one of my TBs who really felt the cold I liked to use leg wraps and a hood with two rugs, one 300 and one 100 if it was very cold. Found using two rugs kept her warmer than one heavy one. The wraps and hood really do make a difference.
in the field my wimp wears a 360g weatherbeeta arion combo and his 200g combo underrug.
in the stable he wears a 400g pessoa stable rug with a 300g underrug.