msklar,

someone has invented that very thing (forget the website of course he sells it for $300ish and it is so easy to make on your own. it is a pole tied down into the ground with ropes (kind of like a tent pole tied down with ropes) and you set that pole to the correct hight/distance form you (depening on how flat/steep you want it) and keep the rope taught throughout the whole swing. i think it is a great idea for any begginer to feel the proper swing plane.

and i don't recomend personally.... but if you want an in to out swing then just put the pole more towards your back foot. but the golf swing is more of an oval than a circle so it is not a perfect thing, but can help you feel a good plane if you need to