It is just an old trash can that we were no longer using. I'll try my best to explain it and the construction. Very simple actually. The pipe you see is the output of the pond pump. It runs from where it enters the filter down to 5 or 6 inches above the bottom. That pipe ends in a T and those run out to almost the edges and at the very end I placed 45 degree elbows pointed in opposite directions which causes a swirl in the water. I placed two or three bricks in the bottom center to hold up a piece of plastic that I cut from a grate that covers florescent lights; the kind made from 1 inch squares and are roughly 48 inches long by 24 (or so) inches wide. The input pipe runs through the center of that and supported by the bricks. On top of that plastic screen I put two layers of filter material. I bought a 3 foot length of it on amazon for 10 bucks or so. On top of that, I put two bags of lava rock that I got from Lowes / Home Depot. and that's the extent of the filter. At the very bottom, on the side you can't see I put a bulkhead fitting for the drain plug and at the top I found another bulkhead fitting that acts as a spout for the filtered water to flow back into the pond. and wha-la. One swirly filter. Not my idea, I pieced together a few things I read on here and put it all together. It works well. The hardest part is keeping the pump clean so the water flows like its supposed to. I have to pull the pump out weekly to clean it. That's a pain in the butt but it works. This summer we're planning to relocate all of our birds and their area and I'll probably dig a much larger pond for them and I'll convert this pond and the one filter into a separate staged filter. Same principals but two stages to add additional filtering. The output of one becomes the input to the second stage and then back into the pond. The larger of the two I'll put aquatic plants in it to really help clarify the water. And provide some tasty treats for our fine feathered friends.
I change water once a month, clean the pump weekly and empty the swirly filter with the water change. It's never crystal clear because of the mud and dirt surrounding the pond but they use it like it's the best game in town. The geese usually hog it and the moment they get out the ducks are running to get their turn.
Let me know if this is confusing or doesn't make sense and I can try to clarify.