I would 100% suggest making a flak vest. T-Shirt material may work if you double it with batting in between, but you'd need more than a single t-shirt worth of material. Ideally you want a stiffer fabric. Not necessarily cordura or canvas but definitely something with a bit more structure and less elasticity. The process I used for mine was to make each panel with two cuts of fabric stacked with a piece of batting. Sew around the edge leaving a hole big enough to pull the piece inside out. Pull it inside out and topstitch. Then assemble the panels. Makes for a sturdy vest you can hang armor off of.
I haven't tried constructing a vest out of a single layer before, but I imagine if you did use a cordura or canvas you could probably get away with a single layer.