I use a seat-stay mounted Secula as my primary, and as back-up I have two seat-stay mountings for 1/2 watt Smarts.
As the Smarts are rarely, if ever, used I load them with Lithium AAA which last significant numbers of years, and improve the waterproofness of the casings with a wrap of insulating tape around the join. Not perfect, but enough layers of redundancy. In fact the Smarts have seen more use being loaned out to other riders who have been 'caught short'. They have the advantage of being also able to be clipped to bag or clothing in extremis.
Unfortunately the rather better designed
Radial Cycles clone of the Smart comes only with a Seatpost mount (and is currently out of stock anyway).