How does one typically do "Old Work" with metal junction boxes?
Look for a "fan" box. they have telescoping arms that expand to secure the box between the ceiling joists. they are designed to hold the weight of a fan, but work well for any heavy fixture.

If your fixture is very light, say under a couple of pounds, simply use a standard metal box with MC clamps and surface ears, and install "hold-it's" on the sides of the box and behind the sheetrock. See   http://meproducts.net/products?subcategory=C42   maybe one of my computer literate buds and find a better pic for you.