Mike, this is an old question that's unfortunately not going to go away. The right answer is to buy a used macro lens that will fit your camera or a new Phoenix/Vivitar 100/3.5. Do it right, don't dick around with second-best solutions that cost as much as doing it right.

Once upon a time there was a photography magazine called Modern Photography. Successor to, IIRC, Minicam Photography. Back before self-focusing cameras came in, MP ran a cycle of educational articles. It took about two years and then repeated.

I mention this because among the articles that showed up every other year was one on whether the cheapskate closeup photographer -- in those days discounts were few and far between and there weren't many macro lenses on the market -- was better off with a set of extension tubes or a set of diopter lenses.

The answer, after exhaustive testing of a fair number of normal lenses for SLRs, was always "it depends on the lens." Some lenses gave better image quality closeup behind a diopter than in front of a tube or two, other gave better image quality on a tube or two than behind a diopter. The only way to know which was best for a lens not in the test was to get tubes and diopters and ask the lens. Go for it! Or don't go for that, do it right.