I used the standard Firestone EPDM 45 mil pond liner. It was pretty easy to work with. Simple 'pavers' from Home Depot served as the place holder for the liner, on top of the block frame. Running down the left you can see the PVC pipe that carries the water flow, to the back wall, over to the right, and up into the 'attic'.
Although I still wasn't sure if my 'skimmer' idea would work, I simply put the 'prefilter' bucket system into the main body of the pond to start. I installed 2 T's into the water line (on the left in picture), which are currently just capped. This will offer diversion outlets for water flow, if I decide to add spitters, or some other water features. I had to get all this work done, and make sure it was working properly, before I finished screening in the left side of the cage. It was MUCH easier working on when I could get in and out of, and around the whole pond area.
Here you can see the 2 barrel bolts on the right of the back 'door; there are 2 more on the other side. Along with the hinged door on the other end of the L shape, this end has a completely removable 'door', again for large access to the cage when/if needed. The top, and all the sides except the screen, are screen 'frames' of wood, which are then screwed onto the main cage frame. If for some reason I do have to open an access, I could just unscrew any of the large panels. The side on the pool screen is simply attached directly to the frame.
Further away view.
Here it is about finished, the left side screen is there. I doubled up the pavers near the splash zone, and built it up even higher in the back side. Later, I would bring the back down to just 2 brick thickness, and the sides at just one. I decided that after fishing some bricks out of the pond that the iguanas had knocked in. My arm does not reach to the bottom of the deep part, without my head getting wet... Note the green 'ladder' material for ease of entry/exit for the turtles!