First thing first;
You're going to get hit VERY often if you decide to adopt a TKD stance and being up real close.
In MMA in general you need to have a good guard that lets you avoid incoming blows, while at the same time defend against grapplers.
What you need is to keep your elbows close to your ribs, your hands about the height of your eyeline, and you back hunched a bit while keeping those abs packed tight.
Remember, you need to look like a tank with a low center of gravity.
You don't need to use this stance all the time (I do however), only when you're up close.
With this type of defense, you only need to duck most of the time, and it's a good base for takedowns when they're least expecting it.
I got two fists.. Don't make me use my head as well!