I manage four panniers sized like yours and more in the original Airporter. I have not seen the new version for comparison. The old price was about $30.

Another duffle that I used previously was the Dana Designs Travel Duffle. Two models: 6500 and 8000 cu inches. I do not know if they are still available.

I use two of them now on multi-month sea kayak tours; one to carry all the food and fuel in dry bags and the other for all camping equipment plus PFDs, four part paddles, clothes, dry suit and everything else. It makes for a convenient carry onto ferry boats on the Inside Passage to Alaska or the Queen Charlottes Islands.