If you want to use metal, you could imitate one of these...



But from what I see, it would probably cost as much in materials as it would to buy the finished product outright. Of course, I spent $175 making a meat grinder I could have bought for $60... and I'd do it again!

Here's an idea... 14 gauge 1" square tubing for the framework, and plywood for walking surface. Self tapping screws penetrate the 14 gauge tubing nicely. You may need to do some welding in some places and utilize bolts in others to facilitate easy folding and storage.