Bags such as this one are often very roomy. This duffle bag is made exclusively from polyester and it has a volume of 20 litres.
From Debenhams
Plain Varsity Barrel Duffle Bag (20 Litres) - Black - One Size
OnBuy
(One Size, Black/Fuchia) Bagbase Plain Varsity Barrel / Duffle Bag (20 Litres)