I give you M734 box & M735 lid - Smartbox
22 litres (measured by filling one with water)
Snap-fit lid makes a waterproof box (easily IP65, & with a sealing gasket would be submersible)
Boxes & lids can stack for storage
Width takes A4 sheet
Size almost exactly at limits for carry on bag
Perfect fit for overhead rack, end racks, under seat, on seat, on most trains
Fits front carrier & rear rack on Brompton (up to 2 high)
Elephant bike 1 each side plus 1-2 across top of rear rack with 1 in front tray/basket
https://www.flickr.com/photos/h52/50774821202
https://www.flickr.com/photos/h52/49880179991
Tweaking the current mould can incorporate spring fit (10mm) clips to latch on to pannier rails & adapted brompton pannier frame
Material polypropylene - can be moulded in range of colours but each would require flushing the mould to change colour (or making unique 'jazz' versions when colours changed) Alternatively produced only in black with all colours of recycled material
Based on prices when in stock perhaps £20-£25 for a box & lid, with options to fit webbing straps to hold down lid, and provide shoulder strap for carrying
Can investigate MOQ for a production run & look at crowdfunding?