You need to copy and improve upon this design.
Then post what you've done.
After using this cart for a couple years now, I will tell you the weakness is the axel. That is the stress point. I did once have the PVC crack on the axel portion when I drop it off my truck. But, being PVC, it was easily fixed.
If being able to fold it is not important, I would use a axel between the 2 wheels and also simply glue the PVC together and not use the wing nuts. But using the wing nuts does allow me to fold it down and store/transport it more easier. But, all and all, it has worked really well.