Is there a tool, or?

I've got like, a countertop, on a stand thing. It is slightly off, in terms of being lined up exactly. ~ so for instance, it's 1.5 in overhang all the way around, but it's like 2# on one of the corners. It's not because it's on there crooked, but because the actual countertop is just, not perfectly square.

I had this idea, like, a clamp, but instead of having pads that clamp down, its got 2 pencils that I can clamp down to just barely able to mark the counter. So, I was thinking, take a 1.5 thick board, and use something like I've described so I can make a nice mark on the top of the counter.

Something like this already exist, or if not, what would you use to make something like that? Or, would you just use a tape measure and a pencil. Thanks for your wisdom, handypeople