Hi Melissa, I checked out the Glencroft site and what an adorable floating house.  Looks similar to a regular bird house but on a large scale.  Have you tried looking online for birdhouse blueprints.  I would think some of the craft sights mights have plans for building a bird house and then you could just nail it to a plank to float on water.  Sorry but I do not know of any sites in the US that make a floating house similar to the one from Glencroft.

If I come across anything similar, I will post it and let you know.  Good luck...seems like a really cool idea for ducks.  Michelle