If you are keen on using a pannier rack, the closest I have seen to a dry bag which maintains the lightness of bikepacking stuff with the security of a pannier is these ones from Alpkit:
Waterproof Panniers and Bikepacking Bags
(there are three sizes available, I found the 20 litre to be ideal and discounted at the moment!)
They are light, simple, secure, cheap, have a slim profile and totally waterproof.
I use a tortec slimline rack on my commuter and it is ace with the above panniers. For a lighter version of traditional pannier set up, I have found this pretty good.