Question about nest boxes for ducks

Hello, my name is JC. I am from southern Illinois. I'm new to the forum today as a member although I've been reading for a few months. I've never owned poultry before but the first of June I received 10 Kahki Campbell ducks in the mail from California. One didn't make it but I have 9 girls who are big and healthy and I'm hoping they will start laying eggs in another couple of months. I am keeping them in an 8X12 horse stall in the barn with straw on the ground where I lock them in at night. In the day I let them out in a pen with grass that's about 35 X 20 surrounded by chain link fence and I have two plastic kiddie pools for them there, they seem to love it. They get All Flock Poultry feed in addition to anything they can forage in the pen which they spend most of the day collecting insects. They seem to be doing great and are healthy but a bit shy. My question is does this setup sound good for ducks as they get larger? Also I would like to know what kind of nest boxes I can make them in the barn? Should I place a raised shelf for them to get up off the floor, or place them down on the ground? Based on what I've read so far I expect them to start laying around November. Any info on nest boxes or suggestiosn for my set up are very very much appreciated.

Thank you!