We have found 10 bags that match these dimensions.
39x29.5x20cm
28L
The bags below are not exactly the size you were looking for, but perhaps you'll come to like them.
39x25x20cm
23L
38x27x13cm
linear: 78cm 16L
35x25x16cm
linear: 76cm 13L
39x28x12cm
13L
30x25x13cm
linear: 68cm 8L
28x19x15cm
~8L
29x20x13cm
linear: 62cm 7L
20x15x8cm
linear: 43cm 2.5L