Scumbag brain
This is my favorite way of studying vocab. I just use 3x5 lined index cards cut in half and then folded. The cards fit in any pocket and don’t take up any room (like a small notebook does, for example). And I can carry only the card I’m studying, instead of having to cary a bunch of other stuff (again, like carrying a notebook).

I fold each card in half because then I can test myself by looking only at the English or only at the foreign language.
Simple and effective. Oh, and it’s obviously super cheap.