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