We have Givi Trekker panniers, one is 33l & the other 46l. We have used this sort of bag in them for 10 years now (photo, Sat Nav & glasses case for size comparison). Found them on E-Bay for a few £'s. They are large enough for us to get sufficient clothes into them for at least 2 weeks (we have done 4 weeks with them once). if you can't get sufficient in them you're taking too much!
They sit nicely in the pannier with room for a pair of shoes, wash bag & a few other small bits.
The pannier isn't opened during the day & anything we will require during the day is in the top box or the tank bag.
I have used the stuff sacks the Upt North posted & they work just as well. If you wish to leave your panniers on the bike overnight it is handy for the bag to have a handle, if you take the panniers off it doesn't matter.
I have had a quick search but can't find anything the same but these would work I think? Or measure the inside of your panniers & add that to a search for "carry on/cabin bag" on E-Bay.