View attachment 1820709 My duck pen is the same size as yours, but my duck house is attached to the outside and opens into the pen. I used pressure-treated 3/4” plywood for my floor, due to local refs requiring a solid surface floor, and painted it with exterior paint.
I wrapped it in 1/4” hardware cloth down low and 1/2 hardware cloth everywhere else. I slanted the floor slightly for drainage and put a no-sill cleanout door at the lowest point, so that if I hose out or dump the kiddie pool, I can open the door and the water drains quickly. Water drains more slowly when the door is closed, which is fine for rain or splashing. I put the pool and water buckets near the door on the low side. Unlike in this picture, I now have the pool next to the cleanout door and the water buckets where the pool was.
My duck house is elevated four inches above the level of the pen floor to keep it dry.
We have many of the predators you mentioned. I put screen door grates over the cleanout door and lower part of the main door and ran a stretch of 24” high 2”x4” welded wire fencing around the rest of the bottom, held away from the hardware cloth by some 2x4 pieces attached vertically to the outside of the pen to make it a little harder for my dogs or other animals to tear into the hardware cloth. I secure my door latches with carabiner clips. Security is a big consideration and you won’t know that you didn’t do enough until those predators show you by killing or injuring your ducks.