So I traced out the t-shirt, made some adjustments, redrew this, tweaked that, and finally I had a pattern I was happy with for the front and back of the tunic. So I grabbed the bolt of muslin I picked up the other day for making mockups of all the pieces, and cut out the back and two front pieces (since it's a wrap front so the collar wraps correctly) and sewed them together to see what I got. Looked pretty good, and fit like the t-shirt on my dressform.
Now for the collar. And that was a trip to figure out because of the three fold look of the collar on the under tunic. I didn't really have a pattern to follow for the collar, so I just had to wing it and see what I came up with! I think I went through 4 collars before I finally got a mockup collar I liked and successfully got it attached to the rest of the under tunic mock up. Looked great on the dress form and fit me great too when I tried it on!
Now for sleeves, since the t-shirt I had used was short sleeves. And I needed fitted extra long sleeves! I laid out some interfacing to start attempting to sketch out a sleeve pattern when an idea hit me, and I grabbed the pattern (Suitability 3409) for the Western Pleasure show shirts I've been making, and pulled out the pattern for the sleeve. It fit perfectly onto the arm hole of my undertunic, and after retracing the sleeve pattern, making some minor adjustments to it, and adding 12 inches to the length to get the gathered look at the wrists on Obi-Wan's under tunic, I finally had sleeves! Now adding 12 inches might be too much, but it didn't look too bad when I tried it on and bunched the sleeves at my wrists, but if they do turn out to be too long, I can always shorten and rehem them. I'd rather have them too long, than too short, because too short means having to make a whole new under tunic.
So I'm really happy with how the under tunic mock up came out! Will work on mock ups of the rest of the outfit, then see about getting the real inner tunic made once I've had a chance to wash and dry the fabric I bought for it!