Mine started as one of those car cover things I got at tractor supply for about 100 bucks. I later bought the end and side pieces for about another 100 bucks.
Being that its white, I get a lot of indirect bright light which the plants seem to love. It is a 10x20.
I also beefed up the frame with conduit pipe and anchors that screw into the ground about 2 feet, so the wind wouldn't blow it away.
Its already too small and I may need to put another one right beside it.
If your getting a 18x20, thats just about the right size. I've got 3 shelves high in mine now and still don't have enough room.