I'll tell you what I do on the log homes that I wire. During rough in, I leave the box projecting 3/4" from the surface of the log. When finishing, I cut a 3/4" thick wood board from the same material as the logs. I make it a little longer and wider than the base of the fixture and round or bevel the corners for looks. I then saw a round hole thru the the board (a little larger than the diameter of the box) on center where the fixture mounting screws will attach to the box. I then draw a pattern of the fixture canopy on a thin piece of sheetmetal, cut it out with snips and saw a round hole (same diameter again) thru it on center with the fixture mounting screws. I then place the board over the round beam and use finishing nails to attach it to the log directly over the box. I then lay the sheetmetal pattern on the board and attach it. The sheet metal will cover any combustible wood that is exposed under the canopy. I then install the light fixture. This makes a fairly good looking installation, and my customers have always accepted it. I know that it sounds pretty "labor intensive", but it's figured in the price. If you've ever wired a log home, you know that a lot of things are "labor intensive". If i was attaching to a round steel column, I would just cut a piece of sheet metal the same pattern as the fixture canopy and use it to close the back of the canopy when attaching the fixture to the box.
hope this helps
steve