I made myself a pair from a 2x tshirt, but it was a big and tall 2x. I added a panel, about 4 inches wide, of stretchy jersey type fabric along the side of the leg. I also made a rolldown waist band of the same fabric as the side panel, rather than using only tshirt, as I found that the rise would be too short, so I think it is absolutely right that a big part is the curve of the armhole being long enough to create an adequate inseam. I also succeeded at making myself more of a pair of long shorts with a regular XL tshirt with the sort of additions.
Hope that made sense...
Afterthought: I am usually a 20 in pants
Absolutely! Thanks a BUNCH!!!