I made this sledge probably 6 years ago out of pressure-treated 2x6's and a hunk of 1/2" UHMW (black) that I got from some guy who had it in his shop but had left it leaning so it bent and was no good for his uses.
The tail gate is simply pinned on top and slide-catched at the bottom.
The great thing about this is that it will haul HUNDREDS of pounds, even with just my ZTR pulling it.
It has ZERO lift-over height, which has come in handy for moving big rocks and stumps - just roll 'em in!
It also does NOT tear up the ground or grass at all, nor does it leave tire tracks.
It IS a bit heavy to dump, but if you built it out of aluminum tubing and a 3/8" or 5/16" UHMW (3/8" will still put up with rocks and not bend too much, which could make tracks when dragged).
Also, it can be moved around easily even when full of stuff by pushing it with your foot, or pulling on the rope. It also can be slid sideways easily to snug it up to a wall or some such.
I want to add some drop-on sides of cheap plywood to make it taller for hauling even MORE leaves in the fall - here in Oregon we have Big Leaf Maples which dump tons of leaves so fall clean-up is a chore.
It's also great for moving stuff like the BarBQ - just roll it in and drive slow.
I added a hook on the end of my ZTR to snag the rope on.
I bent the UHMW by making one saw kerf and then heated it up a bit with a heat gun and screwed it like mad to the 2x6's.