Just saw a headlight at Walmart - runs off 4AA, incandescent appears to be a pr based bulb, supposedly waterproof to 30 feet. Listed at less than $10. Might be a good base for some mods.
I got one from Walmart yesterday. I found it to be pretty big for a headlamp, but it seems well built and plenty waterproof. It has beefy o-rings and a smooth switch. There's also a couple like it that have the batteries in the rear, but I haven't tried those.
As for mods, I went nuts when I first saw it and immediately thought "triple 5 watt luxeon headlamp - one white, one orange, one cyan"
, but I couldn't figure out how I would switch colors.