This is 1 of the rare instances where we will recommend a zoom focus light. It is hard to do better than the Wowtac A3S for $20. The poor amazon reviews don't seem to track with people's experiences here.

The smallest thing I would recommend is the $25 Coast HX5. Its only got 1 mode but the focus isn't so great.

For something a little larger there is the $36 Lumintop Zoom1.