That's a low bar. I hate that it handicaps any light that has a higher mode even if it has a 500 lumen mode. It'd be better to seek a light with the best battery life at 500 lumens.
If I were to build your light, I'd get a light that uses 3x 26650 in parallel, with multiple XM-L2 U2 emitters on Noctigons and AR lenses. The driver would be detuned to 1A. With 4500 mAh batteries it should last over half a day on high.
EDIT: I read it again and see that you're okay with modes limiting output to 500 lumens. That's good. So the Fenix recommended below would be too. Maybe the Trustfire TR-S700 too. The TR-S700 should be even better if modded with XM-L2 U2's on Noctigon's, and might be even better with Q bin SST-90's if you can find a place that sells them.