We use Animal Arks and they're very good. We bought insulated arks which have worked very well not just in the cold but also in the hot weather (yes, we do get some occasionally in Scotland) as they don't get as hot inside as un-insulated ones. If your OH is a chippy he could probably make you a wooden triangular one, couldn't he?
Make sure they have plenty of closable ventilation, make sure they have floors. 8'x4' should be fine for your 2 girls, but you might want 8'x6' if you're thinking of getting more in the future. Animal Arks do very good farrowing arks too, if you do get into breeding.