Today's cart is also a trailer for pulling behind my beach bike, as shown in the photo, however, there is a handle for pulling by hand indicated later in this Instructable.
Link to bike project: http://www.instructables.com/id/EXTREME-FAT-TIRE-BICYCLE/
Step 1: Basic Frame
I figured what the overall dimensions I needed for my large cooler, tackle box and a large bucket for the basic frame rectangle.
The corners were clamped tightly, then drilled and screwed. This seems to be adequate for the connection after several years.
I added a length of angle across the frame near where the axle would go, for added stiffness.
Step 2: Handle
I ran the length of the frame, underneath and then used a conduit bender to get the curvature I needed.
I hammered the conduit a bit flat along the length under the frame so that it would have less of a tendency to roll, then drilled and screwed it to the frame at the front and back.
I used machine screws with wing nuts here so that the handle could be removed for transport.
I added two narrow aluminum straps from the handle to the front frame member as diagonal braces, also attached using machine screws with wing nuts.