One standard solution is to cover all of the nearby walls and ceiling with black. Either paint of matt board.

But like Dan I also use a cold light head. So I've been considering simply going to a fabric store and purchasing a yard of lightweight, tightly-woven, black cloth. Something smooth, like silk, but less expensive. Then cutting it square and draping it over the head during exposures such that it hangs down a little below the negative stage. Simple, easy to do, and probably overkill.

I've also figured this piece of cloth could then do double-duty as a lightweight drape over the bellows of my 8x10 when the sun is shining. (Not that that ever happens in the Seattle/Puget Sound region.) Sort of the inverse problem.