The PDW RADBOT 1000 comes with this rack mount (plus a seat mount AND a seat stay mount; at least mine have & the PDW site states this under the light's "Details" tab). The RADBOT is also a decent tail light.

Some of their other models might also come this way, but the Details on their danger zone light does not list a rack mount, so best check before ordering other models. However, you can buy the rack mount as a part and Planet Bike offers a similar (identical?) one as a part.