I believe if you look at this in a pano you will see a small pinhole at the top. Sometimes this pinhole is bigger or very small. It depends on the scene and how level your camera is.
There is no way to avoid this. You are taking 4 drum shots and when stitched they don't quite cover the whole 180 at the top. You will have to patch this in photoshop.
The bottom part is of course your rotator. These parts of the rotator extend out from your tripod several inches and are captured in the image because of the verticle FOV of 180 this lens has. There is no way to avoid this.
I won't matter what stitching program you use because this will still happen.
There is no majick bullet in panoramic photography. You now have an awsome lens and rotator but this comes with a few things that are minor problems.
If you are looking for a totally automated system that requires no photoshop touchup and will spit out a completed product you will have to go with a one shot system. But then you are stuck with the quality that will produce which is not compariable to the sigma in any way.
You could rotate the lens and shoot a top and then stitch 4 + top. But then you have to content with stitching one more image and deal with the blended edges of that. Which may or may not have some color imbalance in certain situatioins. Then its back to photoshop
There are a couple of simple solutions you can use.
1. Shoot the top and stitch that.
2. patch it in photoshop
3. Limit the verticle FOV to about 175 deg and you won't see the pinhole. NO ONE will notice a loss of 5 deg of FOV. OR you could cover it with a very small cap.
4. For the bottom I recommend you use a tripod cap. Again you could limit the FOV here as well.
This comes down to how "perfect" you need to make your images. Pano perfectionists insist on eleminating all traces of imperfection. This may be necessary in some situations especially when making a "showcase" piece or entering into a contest of some kind.
If you will be doing "production" work for real estate THIS IS NOT NECESSARY.
This is my opinion of this problem........patch the top in photoshop and put a cap on the bottom with your logo and contact info. You NEED the credit. Credit is how you get other jobs. I NEVER publish a pano without my credit on it. That is not for sale. I do share the cap witlh the client when asked by puttiing their logo on the cap with my name around the edge.
Consider what you are charging, how busy you are and whether these minor issues are really worth fixing. The quality you can do with the Sigma is just great. This will out do any low quality providers. But you have to keep in mind the TIME it will take you make a "perfect" image.
I use the Nikkor 10.5mm and shoot 6 only. I don't even shoot the top or bottom. I put a cap at the top and bottom. I have had no complaints in over 200 tours.
One last thing. IF you are doing a "high end" job for a special client. Discuss this issue with them. Find out how perfect they want it. I bet they won't care about having a cap. In fact I bet they will ask to have their logo on the cap. If so charge them a little extra for that. Some images may require perfection but not all of them. Its all relative.
The issue of "perfection" comes up a lot. I started out in perfection mode. But I quickly decided that the amount of "perfection" is directly related to the price.
Do you want to "make money" or "make art". Artists typically starve.