I've never covered a hutch with anything! Now mine are in sheds but before that I used to get a piece of perspex about 3/4 the size of the front mesh bit made up, or to cover the whole mesh part with holes drilled in it for ventilation, so that they could see out but without the rain and wind getting in. Or as a budget version, get hold of some clear plastic sheeting, even freezer bags opened out, and attach it using drawing pins (remember to leave some gap for ventilation. Personally I don't like using tarpaulin etc as they can't see out, and bunnies do like to be nosey, they will almost always choose to sit in the light part rather than in the dark wooden part. As long as it's dry and not draughty (needs to be some ventilation) they are ok with the cold. If the hutches are already felted and in good condition, covering with tarpaulin won't really make a lot of difference. Just make sure there's plenty of bedding and stuff for them to snuggle into. if you want to make it warmer then you could put some bubble wrap over the top (you can buy massive rolls of it online) to insulate it a bit.