Originally, I was planning on using an old shopping cart. I figured the two straight wheels in the back and the two pivoting wheels in the front would provide the most ease of movement in/out of and around my garage/driveway shop areas.
Then it struck me, I was about to toss out an old gas grill that had completely rusted out when I noticed the similar wheel configuration (that's why I don't throw away anything until it is really trash). Also, I often see old gas grills sitting in other people's trash, so they are pretty easy to come by if you need one for this or another project.
Step 1: Strip down grill and fit lower shelf
I removed the entire top portion of the grill which left me with exactly those items.
The metal bottom shelf has a large hole to fit the bottom of the propane tank. This hole need to be covered up to make the bottom shelf usable for storage and prevent the new welding gas tank from falling through.
A 1/2" thick piece of scrap plywood was fit over the bottom shelf to cover the large hole. A smaller hole (6" diameter for the 7 1/4" diameter tank) was cut in the plywood to rest the (rounded) bottom of the welding gas tank into.