If you really don’t want to see anything behind the fixture I would go with a smaller box.
All you need to do is take a small square piece of sheet rock and trace it to the ceiling. Cut the square out and your new electrical box will come with a circular template to make your sheet rock cut. You can attach the new sheet rock to the existing ceiling by putting some wood behind both pieces of sheet rock. I usually use paint sticks. The amount of plaster work you will need to do will be minimal.
+1
If you can hang your own lights you can patch your own walls. It's stupid easy especially if you aren't having to match a texture (like and interior wall).