That might be the best. Look at some of them on here, they are pretty simple compared to chicken coops. The base of my duck's playhouse is a pallet with plywood over top and painted. It wouldn't be hard to build a 4 walled box structure. We were beat after building the chicken coop and I had...
I would just ask them to measure it for you. I would hope they would have bought the medium sized dog house for a 40lb dog.
Chewy.com has this house too and shows all the sizes. The medium size would only give 2.44 sq ft per duck...
How big is the dog house?
Standard ducks are 4sq ft per duck, I am sure calls could have less space since they are smaller. Plus, except to sleep and maybe nest, they won't spend much time in the coop.