Have you tried just an XL from Icebreaker or Smartwool? I ask because according to their size charts, neither should fit me, but I can wear them. Yes, they're a little tight...but since I use them as base layers, I really don't care.

Before turning to wool, I bought "long underwear" tops meant for snowsports. They usually came in foil packages from sporting goods stores, and the fabric was of the stretchy technical/performance variety so fit was never really an issue. Again, they were clingy, but I didn't care because I always had on something (even just a vest sometimes) over them. Costco and Target also usually carry something similar--one of them under the brand "Cool Duds." (Or maybe it's "Warm Duds"...sorry, my brain isn't fully online during the weekend evenings.)

For some size reference: I'm 5'5", 220 (though I've been up to 240-ish with my current wardrobe), an "apple", and most often wear an 18 or 20 in regular clothes.