[Ria P]

I bought thick thermal lined curtains and they made quite a difference and move cages away from windows in the winter.
You could also line halfway up the inside of the cage with boredom breaker mats to keep drafts out.
Hamsters go into their houses or burrow themselves in when it gets cold. A Syrian needs a big house to build a big nest in. I use the inexpensive Ferplast guinea pig houses and leave big heaps of torn into strips plain white toilet paper out for nesting material.
My late Syrian Henry used to insulate the inside walls of his house with an inch thick layer of paper. Hamsters are quite creative in keeping the cold out when given the right kind of supplies.