As the title covers, I'm in search of a tail light that can clip or attach to a seat wedge bag. I'm using the Bontrager Pro Medium QC Seat Pack. I wish to mount a light on that back loop with the B symbol.
I'd prefer something thin and light, yet bright. USB chargeable. Something decent. Something that is designed with this purpose in mind, and not something I rig up. Something that mounts clean, sturdy, flush, and no sag.
So far I've only come across the Lezyne Zecto Drive Taillight that has a clip or tube strap option. Seems good, but I was wondering what my other options might be.
Another option I considered is getting the Bontrager Tail Light Seat Pack Clip and https://www.trekbikes.com/us/en_US/e...?colorCode=red.

It just seems my current tail light on my seat post is somewhat obscured by the tire and bag from the rear.

I don't like seat bag clips. You can't aim the light effectively and the light can fly off if you hit a bump. Try a seatpost clamp or seat stay clamp instead. Personally, I'm a fan of CygoLite taillights (which often come with both seatpost and seat stay mounts), but I'm sure other brands have good products as well.

There a China knock off versions of the Lezyne on eBay, cheap. Sad to admit I have three of them, and they all work great. I assumed they would last a month of so, but they seem better made than my Cateyes - which are probably also made in China....

I have the genuine Lezyne USB for the front, two of them in fact, great flashers. Still flashing after all day rides, 100 charges and still fresh

Cygolite HotShot and Niterider Solas fit the bill. Cygo's are better for straightline visibility, Niterider better for side visibility. The reason is obvious: the niterider red lens wraps around to a greater degree than does the Cygo lens.

Both are available rated at 150 lumens at a great price, usb charging.

These will not unclip from a seatbag unless you ride in conditions that turn your bike into a bucking bronco.

The lights will sag downward a bit on the rear of the bag. I glued a bit of foam behind the clip so that the light sits at about 90 degrees.

No problems with the Cygolite Hotshot 50 on the Serfas Speed Bag light clip. It's relatively lightweight so there isn't much mass to toss it loose. And it runs for a long time per charge. For daylight use you'd want the more powerful Hotshots.

The Blackburn 2'Fer clip holds securely too. It has multiple nibs and hooks to help retain it on various size straps. Doesn't run nearly as long per charge as the Cygolite Hotshot. I use it as a helmet light.

Not secure: The Planet Bike Rack Blinky 5. Popped loose first ride. Good light for the money but secure only on the provided mount. The weight of a pair of AAA batteries is too much for a fabric loop.

Great suggestions folks. So far the number one contender is the Hotshot 150. It will be used mostly for daytime flashing on my hybrid. The Niterider looks sweet too, but a bit too bulky to be hanging from a bag IMO.

The problem with "sag" is typically the bag and the clip strap sagging, not the light. You may need to put something in the bag to keep the back of the bag perpendicular with the ground. I've done this a couple of times and it worked OK. Many of the lights with clips that will go over the strap, can bounce off the strap on a big bump. This can be fixed by stretching an elastic over the top of the strap clip and under the bag strap. It holds the clip down on the strap. I've done this on my bikes and other riders bikes. If there is any way to mount the light to something solid, try it.

I'm fairly impressed with the Bontrager Flare R and RT taillights. USB rechargeable, visible day or night, and with a variety of mounting clips available. I install them on the rear of the trunk back (flat mount), and the angled seat stay. They work very well. Can't say that I've tried the "bag" clip variation. I've never had too much trust in how modestly those things clamp on. Not positive enough on the attachment, for my money.

I have this light and absolutely love it. Had a guy stop me and ask what kind of light it was because he could see me more than half a mile away in full sunlight.

Here's a useful little tool: The Best Bike Taillights @ Bike Light Database.