I made this bottom plate extender for my hand truck mainly to handle larger loads of firewood and moving boxes, etc. You can see it is nothing special, just some rough maple boards from a skid. Even the skid's nails were reused to assemble it.
Step 1: In Use
The extender was made primarily for moving firewood on my hand truck as per the photo. It works well for moving boxes and many other things as well.
I cannot take credit for this idea. I saw a gent bringing in potato chips into a store and he had a similar device on his hand truck to move many boxes at once. His was different in that it was a swing down tubing platform, attached permanently, but swung up out of the way when not needed. I too thought about making a permanent tubing version, but have not as my crude solution works well for the number of times I need it.
If on the other hand you prefer to just buy this accessory, or make a permanent type, Grainger has them.