There are a few good lights that are specifically designed for rack mounting.

Portland Design Works makes a rack mount bracket that sells for $8 that I think will allow any of their lights to be rack mounted. Those are all decent lights, the Danger Zone, the Radbot and the Red Planet.

Cygolite makes a hotshot rack mount bracket.

When I say the Hotshot isn't my favorite, keep in mind that it's still a pretty good light, at one point it was probably the best < $50 taillight. It's just that IMO right now you can do better for the same money. That doesn't mean it won't serve you well.

I kind of like having multiple kinds of lights mounted at once.