Hah, I had avoided undershirts for quite some time now but I sometimes like wearing a shirt mostly unbuttoned with a deep v-neck/tank under it or something. It obviously didn't work here for a couple reasons, but this is sort of what I was imagining:
Warning: Spoiler! (Click to show)
Shirt's too formal, though, and undershirt too undershirt-y. As for the pants, it's a little tricky as they have a harness inside but I think I will hem them a couple inches. Again, thanks for feedbacks.