Steiner appears to be clearing out their MK7 (350 lumen) shotgun lights. Cheap. Like $45 cheap if you buy two or more, with free shipping. They are on the Steiner-optics website.
The design is a little odd but the quality of build is top notch. Originally listed at +$200. No wonder nobody bought them.
They mount in place of the magazine end cap. So you'd best be happy with the factory capacity of your shotgun. Short stocks and longer than average arm length really helps here also.
The switch is on or off, no momentary feature.
If you can deal with the downsides, it really is a super clean, super cheap solution for the Beretta 1301, Remington 870, Mossberg 500, and Benelli Nova/Beretta M4.