It might not be quite the answer you’re looking for, but generous fit around shapewear, torsos, and butts was a key feature of centuries of historical European men’s shirts. If your comrade can get down with ‘em stylistically, there’s a lot more variety than just stereotypical flouncy pirate shirts. For most of their heyday, they were typically worn tucked deep down in lieu of underwear.

I’m having a hard time tracking down a broad lesson on just historical base-layer linen shirts, but excellent historical gear merchant and YouTuber Townsends can show you some silhouettes. Don’t be freaked by what they’re charging, it’s premium priced srs bsns reenactment stuff. They made a little explainer video: https://www.youtube.com/watch?v=_FbwRSfedoE

The garment is reputedly a great beginner sewing project, or easy to crank out if you know someone really stitchy. Victorians used to buy them by the half-dozen or dozen. Here is a creator making one who mentions having top surgery: https://www.youtube.com/watch?v=lORt8kIwsNw