"What you really need is some yew bark," James ploughed on, thoughtfully. "There are some trees on the grounds somewhere, if I remember rightly. Yew deflects magic back, rather than trying to absorb it, making it a great defensive material against average-level spellwork. Sirius built your Mum and I a sort of palisade wall that we could concertina down and carry around with us. I'll get him to send you the schematics. Very handy for camping, or if you just fancy a roll around at the side of a motorway or something. Your Mum was big into powerful Muggle cars ... Hermione is Muggleborn, maybe it's a kinky thing for Muggle girls or something, you'll have to ask her ... long-distance driving with your Mum was often an interesting experience ..."