It's just sitting on top, I didn't want to modify the deck too much in case the new pool didn't work out. I'd never added a drain to anything before and so thought I'd mess it up or it would leak. They have a step to get into the pool and a clear stick on stair tread on the lip of the pool so they can grip on to get out.
We are lucky our ducks are pretty good with small steps and ramps, much more agile than I'd expected before we got ducks.
After building this first setup I have a better idea of what we do and don't want for an enclosure in the future. A coop and run that we (humans) can stand up and move around in is at the top of the list