On several rides lately I would have liked to have had my Nikon camera. Would be ideal to carry waterproof jacket and snacks.

Specs I would like are:

Waterproof
Able to open one handed
Able to open and close while on the move

Cheers

Not sure how waterproof you want, but your other two criteria are inevitably going to compromise it. I assume you want “fairly rainproof”.

Outershell gets good reviews:
https://theradavist.com/2017/01/outershell-makes-the-best-on-the-bike-camera-bag/

The other option is to get a camera bag and convert it to Klickfix:

Turn A Camera Bag Into a Handlebar Bag

I’ve done this myself and it’s easy (though I didn’t use a camera bag specifically, and I wasn’t anticipating too much weight so didn’t incorporate a stiffening plate). The bar bracket is about £12 and the adapter plate is about £8.

How big is the camera? An Alpkit Stem Cell is good for a reasonable compact or small mirrorless camera. Quite easy to open while riding. The current version is just a drawstring closure, so probably not really waterproof.

Bez, really liking that Outershell bag, especially the fact that it opens while I’m riding, nice design.

The Road Runner Jammer bag would suit. They have a waterproof liner which really does work and are easy to access.

@fudge9202 is this for on or off road?

Ortlieb Ultimate Six Plus meets all your criteria, it’s a top quality bit of kit that works really well. Have had mine for a few years now and have no complaints.

@damascus for use on and off road( primarily on gravel on my Salsa Vaya)

The Outershell is good, but won’t take a big camera & a jacket. It will hold a Fuji XT1 with 18-135, but that’s it.
Ortlieb’s are bigger, but the rigid mount can wobble a bit on rough roads.

@kiwijohn cheers for that detail my camera is large dslr and would want space for a jacket and snacks so the Outershell is a no go, pity🙁