I'm in the slow process of building mine.
I'm levelling the ground right now and waiting for a break in the rain and leaf-raking season.
I plan on setting 4 pier blocks holding 4x4 beams, the plank off a floor slatted with 1x8's for air circulation.
On the corners, putting up 4 more 4x4's so that they support a roof sloping towards the back. Longer posts in front.
These I'll connect at the tops with 2x6's and support the posts with 2x4's, which will also help keep the weight of the wood from bulging out the siding.
The roof will have 2x4's running front-to-back to support the plywood in case of a heavy snow. Then this will be capped w/ plywood, tar papered and shingled.
The siding will be T-1-11 for water repellency & painted to match the house. These panels will be cut to fit and screwed on with decking screws. Plywood just doesn't hold up. I built enough forts as a kid to know that quite well.
For the open front, Im trying to decide whether to run a taut-line across the top and hang a sort of "shower curtain" type cover (imagine a tarp with a thin wire threaded thru the grommets), or if i should install a track up above and make a sort of slider out of the scrap wood.
The big part is the dimensions. That determines everything. Unfortunately I can't measure off the pad until it's all levelled and compacted. And I cant really get started on that until leaf season's over.