Step 5: The frame
I added a little standing platform on both sides just to make it easier to get on and off - but it isn't necessary
2x4 studs or other common materials will work fine - you certainly don't need to use hardwood like is pictured in mine
You will also want a way of pinning the axles so that they don't turn or try working their way out while moving.
Note the notch in the middle cross pieces to accommodate the webbing