OK, so I was thinking about the wheels… my fold up cart measures 25″ in length, not an option, too long.

I remembered a wheeled shopping bag/cart I bought a couple of years ago, never used it. Found it in the sewing room closet.  Too long and narrow as it was to use as a carry-on, so I took it apart, cut off three of the four sides, added a “sleeve” for a bungy cord to slide through at the bottom, a couple of rings for the cord to hook to at the top, and the velcro from the cut away front to the back so I can fold it all up and tuck it away. Here it is folded up… With the wheels opened out it works perfectly with the carry-on, and I can slip my backpack in as well – better then on my small hard-side bag! 

This is the back:

How perfect is this?