Nice, probably too much for him though.Ortlieb City, if you can find them at a decent price....
There are two types of clip even on the same model. That's not all that important until you come to repair one so, if you do decide on Ortlieb then make a note of which clip system you got.The wealth of different models for the Back Rollers is confusing but I expect they're all basically the same.
https://www.amazon.co.uk/dp/B01LZKFADY/ref=cm_sw_r_cp_apa_L-H3zb8JJ1NF3
I have the 2016 models. Rolltop fastening with a lid makes them waterproof. Mine are actually airtight for brief periods. Nothing's getting in.
Plenty big enough and they'll do left or right just by tweaking the clip positions.
Heavy though.