Bag for use on Small Wheel folder

This is my new cycle – an Axon Rides e-folder.

I am looking for a casual bag that I can attach to either handlebars or seatpost (or seat), and usa for casual carrying of bits and pieces eg fish and chips on the way home.

If it was a Brompton at 3x the price, I could use their system.

I have a click-on Bikehut small (5l?) handlebar bag which is great for what it does, a traditional market trader over the shoulder fabric money bag, and a laptop backpack.

Does anyone have suggestions for something perhaps fabric which can be attached and unattached very easily to handlebars, seat or seatpost? 

Plan B may be to start looking for a lightweight front rack which does not interfere with the folding mechanism.

Thanks