TopHat is a derivative of XCSoar, somewhat simplified / streamlined. I use it on a Nook e-reader and love it. (Thank you Robert Dunning!) The Nook's high-contrast e-ink display is easily visible in sunlight, and it is large, meaning easy viewing and finger tapping (of the non-rhythmic kind). It does take some baby-ing to make sure it works right: fully charge the Nook before flying, don't let it overheat in the sun, be careful not to break the micro-USB connector (speaking from experience :-), and so forth. I use it with a USB GPS "puck", an OTG Y cable, and a 12V-5V converter.