It's easy to improvise a panini grill on the stovetop.
After assembling a sandwich and spreading butter on the outside of the bread, place it in a large nonstick skillet. Place a piece of foil and a heavy skillet over the sandwich, and flatten it.
You can put a large can in the top skillet for added weight, if necessary.
Cook as directed, until the bottom side is golden brown. Turn the sandwich, reassemble the second skillet and cook until browned.