I just looked into windowed mode, and at first it was really confusing trying to figure out what's going on. But I figured it out.
First, here's a baseline shot at 1600x1200 to show what it's supposed to look like.
Now, it turns out, that in windowed mode, your desktop resolution affects the aspect ratio. I use 1920x1200, and here's what it looks like in default windowed mode on my desktop:
It looks like it was stretched to 16:10 and then the sides cut off to make it 4:3 again. Strange!
In order for Windowed mode to look right, you *must* have your desktop resolution set to a 4:3 mode. Here's that same window when my desktop res is 1600x1200:
Anyway, now that the aspect is correct, you can, in fact, make it hor + by horizontally stretching the window.
However - keep in mind that my desktop is set to a 4:3 resolution and is pillarboxed, so I'm not getting the full use of my screen's width. And since the window needs to be "shorter" than 4:3 in order for the hor + to work, I don't get the full use of my screen's height either. In order for hor + to work, I need to set it up so the screen looks like this:
I don't think I can endorse a hor + solution that puts a small window in the middle of the screen.