I like to watch eBay for the occasional one-off item from brands like Platatac, much as it pains me when I don't have the disposable income, or when out-bid. However, this is one such item.
This is a "Lightweight Long Sleeve" top, from Platatac. Made from 100% cotton, it is a lot like a very much cut-down version of their CUTS pattern shirts, with none of the frills, just similar lines. I really liked that even in only a L size, which meant my super-long arms run a bit short, the waist length is super long, well below my hips.
This means it sticks in my pants no matter what I'm up to, keeping my skin covered up in all circumstances, like a base-layer should.
Being a cotton garment, you need to consider the risks of wearing it in cold and wet survival situations but that can just be a matter of good planning and situational awareness.
One of the things I rerally liked about this design (and this follows with the CUTS designs too), are the high mandarin collar gives great protection from both the elements (sun/wind/sand/grit) but also shields against chafing and rubbing when wearing harnesses, plate carriers and tightly strapped packs.
The chest-level zipper, lets you vent, when you aren't worried about covering up as much, and frankly, lets your inner "hello ladies" out.
When I need to either block the sun, or keep warm in the cold and dry, this is the kind of thing I look to have as a base-layer. Keeps we draft-free, nonrestrictive and takes up little to no room for adding more layers.