I only have a minute to respond, and I was only able to skim the thread, but wanted to share my recipe for (relatively) inexpensive & easy but predator-proof enclosure:
Dog kennel.
I look on craigslist and buy used chain link dog kennels for under $200 (and now I've got a resource for scratch n dent kennels for $145 for a 10 x 10 enclosure with gate). I then lay wire mesh of some sort across the area where I'm going to place the kennel. I set the kennel up on top of the wire, and secure the wire to the kennel with cable ties (cheap at home depot type stores). Then I lay more wire mesh across the top, with boards or rods or cedar sapling trunks across the top for support, and likewise secure it with cable ties (if you want to save yourself a couple bucks for cable ties, you can use wire you have lying around, but it's much more of a pain to do). Then I lay wood chips or dirt or whatever I have on top of the wire to save their feet, and straw on top of that. For shelter, I have a home-built duck house, but they never use it. In the future, I plan to use dog igloos purchased used on craigslist--much easier, and cheaper too, once you factor in the lumber and hardware for the silly house they never use.
For young ducks, and to protect from raccoons that might reach through the chain link after your ducks, you can secure hardware cloth (not really cloth at all--it's a fine-mesh wire fencing material often used for small animal cages) to the bottom couple feet of the enclosure.
Congrats on your babies. They are ADORABLE.