I've just had a go at setting this up with my 3x3 tarp.
I like the set up. The 2nd walking pole does create more space inside.
I used bungees to hold the door back, but the one thing that I had trouble with was getting the first walking pole to stay put, so I kept extending it to make it tighter, then when I looked at the reinforcement point on the tarp that I was using I saw that it had put a fair bit of strain on the stitching for the attachment points with the loops pulling away from the tarp by a couple of mm. I re-fixed it with epoxy, hopefully it'll hold.
I found that by giving the pole a good footing into the ground and by looping a piece of inner-tube to hold the tarp to the pole that it took some of the strain off the top and worked out well.
After a little bit of tinkering with the pegs to straighten things out it was looking pretty good.
I'm planning to use this set up next month in Wales. Hopefully it won't cave in on me under the weight of the rain!