This is not my design. It's the original henspa. I have built a copy cat for my inlaws with some changes, but they are minor and construction related. The overall design I find leaves very little need for improvement.
The egg rollout works great, especially with sawdust down there to catch the eggs. Currently, it is bare so the eggs roll over to the corner. I've had one egg break in the few years I've been using the thing. And that egg rolled over into a brick I had sitting there for some reason.
The carpet stuff I've no idea where it came from, but it looks like an astroturf like material. The unit I made I used the indoor/outdoor carpeting from Lowes. That worked, but not as well because it's not as stiff, so it slips and bunches up at the bottom.
Only change I'd make with the nest box design really is to lower th egg rollout opening. The current flock has two birds that spend time head down, looking out that window. And I've had a little egg pecking as a result. They can't eat the eggs as they roll away as soon as they peck at it, but they've damaged a few from the pecking.