Limited budget: -- duck cloth isn't as expensive as you think it is, especially with a 40 or 60% off joann's coupon which they run all the time -- sacrifice 1 t-shirt to make the pattern, by a couple of yards of duck cloth to make the vest, and absent the expense you would have sacrificed on t-shirt 2, the duck cloth doesn't cost you very much at all, and you'll be much happier. The biggest problem with 2 t-shirts is that your plates are going to sag and bunch together. It needs to be a stiffer material to work.
My flak vest and kama are a set of old black out curtains though, so you might get lucky at a thrift store for material too. My kit is a budget conscious kit too.