Up early and waiting for the day to warm up so knocked up another 'Beebarrow' from a frame I was given last week. You could call it the Mk IV but only because of the plethora of different barrow chassis design out there which then dictates what you can do it, but I have found in most cases a bed footprint capable of holding a single box is the most stable for wheeling, then, depending on how robust the frame is you can probably transport a stack of three or four filled supers. This one will handle four easily. The second one is exceedingly light and transportable but a wee bit 'wobbly.
I have two barrows with a flat bed long enough to take two boxes, but again four filled supers seems to be the optimum payload.