"Re: "If your groundsheet touches this, the water flows right onto it. You could wake up to a gallon of water on your floor in no time." He makes a bathtub ground sheet in the Twin / Long size, $120."
I was referring to the cuben groundsheet. Do not think that it is sturdy such that you cannot EASILY push it into mesh on the rear side with water trickling down it. It is not exactly taught, and is only inches away from the mesh. It may almost lay against it even without anyone in the tent.
If you have ever had to camp where you dont have a flat spot, you know you will end up on the downhill side of your tent. Especially with a slipper cuben floor. You do have a high potential to slide downhill and push your groundsheet edge into the mesh during the night. If its raining, you can end up wicking water onto your floor.
Again, its a great shelter , primarily due to the unbeatable low wt for 2 people. When it gets tight with 2 people, you do have to be careful in hard rain near the rear side. Pitching low to ground, can help this , at expense of headroom and ventillation,(and a good pitch). Of course as you pitch it lower, you lose bathtub floor-ness too.
I can think of several ways to improve the hex twin. But they all would add expense, and weight, which is not the goal of THIS shelter.