Keeping warm in winter

We are doing some DIY on our wendy houses this weekend and I wanted to start thinking about winter. Snowdrop needs to be kept warm but I would prefere not to bring her in as she loves being outside. She has her 6x4 wendy house which has a 5x2 hutch inside- I will stuff this with hay for snuggling in but was also thinking of lining it with fleece material?

Anyone got any other ideas?

You could line the inside of the wendy house with something like ply (doesn't have to be very thick) and squeeze rolls of loft insulation between the wendy house wall and the ply - that would then be a bit like cavity wall and loft insulation for the whole thing

I lined Sooty's shed with vetbed on the floor, cost a bomb but kept it nice and cosy.