Update:
The glass block door has now been made as small as the opaque block door! It is still (and likely will always be) slower than the other, but it now fits in an 8x10 space. This smaller design is available in the updated world download, and there are two photos of the whole design added to this project page. I hope you like it!

More pictures can be found here!

These designs are both 2x2 piston doors, flush and seamless. There are two of them, because one is a standard door (opaque and lever based) which is more compact, and the other is a glass door (controlled by a button) which is slightly larger than the first. They are both fairly simple in design, and can both be found in the world linked below! Feel free to take a look, and let me know what you think. I believe to have made them as small as possible, but let me know if you figure out how to make them smaller so I can admire your work!

Have some gifs of both working: