Here you go!
The design is a bit crude, but the goal was to get the structure's dimensions right. And it worked!
Try throwing render() around heavy blocks of code. That should speed up successive renders quite a bit.
Have you tried freecad? I think it uses openscad as backend but has a slightly better (and faster) UI.
It uses Open Cascade: https://www.opencascade.com/
I'd be very curious to see a couple screenshot of your project! I used Google Sketchup to do landscaping last year around my house; I wonder how this would compare...
sketchup plan: https://goo.gl/photos/Qc5mEnFiFm9UWBvv8 reality: https://goo.gl/photos/Gn66MSc8UPshR3CC8