I also tried to find someone willing to sell me Harp uke plans a while ago and drew a blank pretty quickly.
The only commercially available solution was templates from
http://www.waldronmusic.com/productlist.php?type=&model=Harp+Ukulele&cat=32 but cost of import and general laziness resulted in me shelving the idea.
I plan to attempt a Harp uke of some form in the future but will likely just take a plan of a standard uke and improvise the Harp part using old fashioned guesswork and a lot of swearing.
I was never quite sure how long to make the Harp strings. Not sure if that is me being daft or if there are rules similar to normal scale lengths.
I reckon Harp ukes are a bit of fun/folly so if you want to build one then go for it ... What's the worst that can happen?
Good luck and report back if you have a go