We present a method for rapidly prototyping interactive robot skins using flexible 3D printed material and analogue air pressure sensors. We describe a set of building blocks for presenting affordances for different manipulations (twist, bend, stretch, etc.). Each building block is a hollow air chamber that can be printed as an integral part of the skin to easily add sensing capabilities over any broad area. Changes in volume caused by manipulating the chambers are captured using air pressure sensors; the sensors can be plugged in and removed, allowing rapid iteration on new designs. We demonstrate our method by prototyping three robot skins that attach to the Keepon Pro armature. With fully operational robot skins, we can study the dependencies between appearance, movement, and interactions at a deeper level than would previously be possible at the concept stage.