rucksack on a brompton rear rack

Anyone got pics of a rucksack (45 litre or there abouts) transported on the
brompton rear rack.

1. Have a rucksack already.
2. I have a rear rack already.
3. I want to walk with the rucksack on back.

I know of people talking about using releasable cable ties to fasten botton
of rucksack to carrier and top of rucksack to seat.

But would bungees be better than cable ties?

I hear people talk about this but a few pics would be nice.
Stuart.