If you're ok with changing a AA battery, and don't need a flashing mode, I love the Busch & Müller Toplight Line Permanent -
Busch & Müller Toplight Line Permanent & Senso - Harris Cyclery bicycle shop - West Newton, Massachusetts
It puts a straight line of light across the back of your bike which is the best non-blinking solution I've seen by far. Bolts onto your rack. Drawbacks are that it doesn't blink, and that it requires taking the AA battery in and out to recharge it.
I needed to buy a light for a family member, where I'd put a USB charger next to the light so they could plug it in after every ride and not end up with a dead battery because they forgot to recharge it. It also need to bolt onto the back of the rack, because they had a trunk bag on their bag.
I got them the Planet Bike Blinky Superflash USB 0.5W Tail Light and it's work very well for that:
https://www.amazon.com/Planet-Bike-B...lanet+bike+usb
You also need the planet bike rack mount -
https://www.amazon.com/Planet-Bike-T...ike+rack+mount
And if you have a modern cell charger, the mico usb to mini usb adapter:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
Buying all this stuff has been kind of annoying, but after that it's been rock solid reliable. Because it's all one piece the light cannot have the back fall off (something I've heard can happen with the AAA battery version). It locks into place well on the back of the bike. Since it's left plugged in it's always charged when they go riding. My only complaint is that it's flashing mode is kind of obnoxious, so it has to be left on steady for group rides. But it's been very reliable and doesn't jump off the bike.
I don't know of a light with a blink mode any more that bolts on directly. Planet Bike used to sell a rack blinky, but they discontinued it and replaced it with another similar looking model which requires the adapter to use with a rack. It wasn't a great model anyways - I had issues with the batteries jiggling loose inside the light and it stopping working (without me knowing) in the middle of a ride, so I wouldn't recommend it. That's not an issue with the usb version.