Best thing to do is to have a lumber yard/portable building place make one and drop it where you want it.
I would have come out at about 1/2 of what I paid to have a 10x20 made for me.
And really don't need a plan, if you have someone that has carpenter skills.
Here are quite a few images to look at, pick one you like and frame it up, after deciding if you want it to be movable or not of course. https://www.google.com/search?q=Hors...w=1366&bih=575
But trust me, much less trouble to see if a lumber yard around you will just build you one.