shed

Looking for something that will hold a jacket and a small lock.

Don't need to carry any tools since my Trek Domane has room for inner tube, CO2 and tools in the down tube.

Any recs appreciated. Lots of Chinese stuff on Amazon, but I just don't trust the reviews given how many fake reviews there are.

oris

Why not go down to your local bike shop and see what you will work for you? I would think the average saddle bag would work for this application assuming you are using a packable jacket. Amazon tends to be a crapshoot on what you get.

base2 

Any ol' generic saddle bag would work for keys, a light duty lock, and small fix-it kit. Larger is better than smaller, but you're not going to fit your whole life in there no matter what you do or how big you go. (Revelate Designs makes super large ones high quality ones for camping, even.) So grab a cable lock, your wallet, bus pass, maybe a spare tube & test fit, test fit, test fit until you find one that makes you comfortable. Then go with that one.

I like EO Gear top tube bags. Everything including riding snacks, car keys & credit cards are kept safe readily available inside zippered pouch. I'd rather save the area behind the seat post for water bottles & tail lights, but that's just me. There is no wrong way to do it.

To answer recommendations directly: I'm impressed with EO Gear & Revelate Designs & would look there. High quality & durable stuff. Being made in the USA is just a bonus, if that sort of thing is important to you. But, a Profile Designs saddle bag will do in a pinch & last you long enough if an extra dollar or two is going to break the bank. I never liked digging for whatever thing I needed that was inevitably in the bottom, of a saddle bag but never did it cause any more than a minor annoyance either. The availability of places to put my taillight, for me, became the issue away detracting from any saddle bag.

Oh, edit: Ortleib makes good saddle bags as well. They have a bracket that attaches to the saddle rails & the waterproof (actually true: waterproofed) bag installs to the bracket clip. It's secure and easily removable so there is no digging for whatever settled to the bottom. You can detach & dump it right out! Though the Micro is a bit too small to hold a lock, it'll still hold a multi-tool, patches, CO2, credit card, a car key, & won't block your tail light & the Saddle Bag 2 is 2x-3x as big & wouldn't compromise anything! If you're stuck on a saddlebag as your only possible option, it'd be hard to beat an Ortleib. Otherwise, the afore mentioned budget friendly Profile Design is still a reasonably decent choice.

Good luck.

dr_atom

I also didn't trust the variety of options on Amazon, so ended up going with a 9 liter Apidura Expedition Saddle Pack. It might be bigger than you need, but it rolls up to only be as large as the things contained within it. I use it to avoid having to wear a backpack while biking to work. I can fit a fair bit in it if it's all somewhat compressible (my small tool bag, pants, a polo shirt, a few other random clothing items, a small lunch in a bag..). Quality is great, really have loved having it and am not missing wearing a backpack.

surak

+1 to base2. I have the 11 L Ortlieb Seat-Pack, which I bought to carry clothes on my commute but can roll up to quite a bit smaller most of the time. In fact, the only time I used even close to that capacity was when I carried a full change of clothes plus rain gear to ride STP in one day, so it'd be overkill for you. But depending on the shape of your lock, the Saddle-Bag Two might be enough.

Carbonfiberboy 

I've been using an Orlieb saddle bag for many years, a little smaller than the 4L Saddle Bag Two. It holds enough stuff to ride a 400k with controls every 50 miles, enough to ride STP carrying everything including all my food, 150 miles rides in the mountains, etc. My gear is very compact though. My jacket is about the size of a Navel orange. Usually it's almost empty, but I don't care. If one did care, Ortlieb has smaller bags which fit on the identical mount so you can snap on the bag you want that day.

Marylander

I've got the Ortlieb Saddle-Bag Two 1.6L and really like it a lot. Sounds like it might be just big enough for what you're wanting.