I haven’t seen much 18W charging from the smaller banks, but I use this one, and find it pretty convenient: www.eldirekte.no/…/p1001029953 (couldn’t find a link in english, sorry. Google translate is your friend)
My employer bought a bunch of them to hand out to everyone. I believe it’s the largest battery allowed on planes, and as we’re often in transit it comes in really handy. The formfactor is small enough to fit 8n my pocket.