I use a cheap Beam rack (£10 off ebay IIRC) just clamped on the seatpost and an 8L Alpkit Airlock extra (under a tenner) which holds my clothes rolled up and some lunch (I leave shoes and shower stuff in the office), I also have a 20L Airlock extra Dual if I need to take more stuff…
I did buy a 13L Airlock taper expecting to use it lashed up under the saddle for commuting and a bit of light touring but I’ve been a little disappointed by it, it really needs a ~£70+ harness to be any use…
Dry bags are good because they keep stuff dry (Shocker), are available in various sizes, can be rolled up nice and tight to keep stable, and most importantly they’re relatively cheap… bungied/clipped on a beam rack or on the bars it’s a pretty efficient way to get stuff to/from work…
Edit:
Here you go:
That image is from Oct 2015 so i’ve been using that setup for at least the last couple of years without issue, that same bag is sat next to me now, it’s looking a bit shabby but it still kept my stuff dry riding in the wet on Wednesday…