Aluminum bar isn't the way to go. Steel yes. It needs to be very straight, true, smooth, and the diameter needs to be accurate all the way down. Also, make sure you really need to lap the rings. With very high end rings (Seekins, American Rifle Co., etc...) I've found that they require no lapping. Cheaper rings sometimes don't require lapping, sometimes they require a little, sometimes they require a lot or are too far out to be fixed with lapping.
--Wintermute