A slide electric ukulele would be really easy to build, I have often thought of doing one on a skateboard deck. Problem is most folks who have spent the time learning the nuances of slide (I come from a place where I have watched a man practice Lap, Pedal, Weissenborn and Dobro for my entire life...I call him dad) don't want to be limited by four strings.
That said Mya Moe made one for a while and no longer does...it never really caught on I reckon. Here is a video of it.
I may whip up a 4 string open C Uke steel this summer...because this thread has me thinking. Be on the watch. Really though it would not be an ukulele...but rather a short scale lap steel. All my dad's friends are going to ask "why?"