Alpine ducks

I noticed that pair of Mexican Ducks is still hanging around (see post of Feb 2, 2011). We have a pair of domestic ducks also, and we've found quite a few duck eggs in the ponds. I don't know which ducks laid the eggs. So today I tried to construct my first duck nest tunnel. We'll just have to wait and see what happens. I know next to nothing about it but it would be so cool to have baby Mexican Ducks running around.

Any advice on how I can improve the nest will be appreciated.