Haha, I have a trench coat, medal, and my big white scarf from when I was Russia, so sometimes when it's cold and rainy, I wear it all and wonder if anyone suspects.
Well Mathias's shirt doesn't seem so hard though. A button-up shirt (maybe a thicker, military-grade one) with a belt and some cardboard and cloth for the shoulder pads. It's amazing how much minor adjustments can change normal clothes.
And yeah, I do that a lot too XD Although I tend to pick clothes I own or see around me just so that I can have a ready reference for them. Although I often accidentally give characters clothes I own. Like it took me forever to realize that Vya's boots were just like my favorite pair (which I have recently lost somehow) So now I just pretend he accidentally bought women's boots without realizing it XD (they were probably all that fit his tiny feet)