Interacting with touch screen-based computing devices can be difficult for individuals with mobility impairments that affect their hands, arms, neck, or head. These problems may be especially difficult for power wheelchair users, as the frame of their wheelchair may obstruct the users' range of motion and reduce their ability to reach objects in the environment. The concept of chairable input devices refers to input devices that are designed to fit with the form of an individual's wheelchair, much like wearable technology fits with an individual's clothing. In this paper, we introduce a new chairable input device, the Gest-Rest, which provides a pressure-sensitive input surface that fits over a standard power wheelchair armrest. The Gest-Rest enables users to perform traditional touch screen gestures, such as press and flick, as well as pressure-based gestures such as squeezing and punching. The Gest-Rest enables multiple inputs, unlike most switches, and does not substantially change the shape of the wheelchair armrest. We present a formative evaluation in which nine wheelchair users and three clinicians tested multiple gestures using the Gest-Rest prototype, and provided recommendations for integrating the Gest-Rest with computing applications. Our study showed that our motor impaired participants were each able to perform multiple gestures using the prototype, but had some difficulty with the pre-set sensitivity settings, and would thus benefit from a more robust gesture recognizer.