Could always go with an "AusPack":
Australian military back pack used in the Vietnam era, commonly refered to as the "Aust Pack"
View attachment 4068
- Overall size : 50 x 35 x 20cm approx.
- 2 top side pockets 22 x 17 x 6cm approx.
- Top compartment 23 x 25 x 17cm approx.
- Bottom compartment 34 x 25 x 17cm approx.
That brings back some fond, but painful, memories. It was my first all-purpose bushwalking rucksack back in the 80's - used it till it fell apart and had to be returned to the Crown. I was able to fit 10-days worth of gear into it. Good points - slimlines and hard-wearing. Bad points - very uncomfortable 'frame', suspect waterproofness (lid). Time have changed and there are much better designs and materials available. Though, for a day-walk pack, it would be a good choice - no waist belt though.