Water Gardening: Water garden inside?, 0 by

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If you have never seen an undergravel filter, go to any store that sells aquariums and look in the filter section just to get an idea of what I am talking about. It's a piece of plastic with slits in it that hold gravel above a space over the aquarium floor. The idea is that the water is pulled up from the bottom of the tank, through the gravel. The gravel is home to nitrifying bacteria that break down waste products from the fish.
I have a large one attached to my pond, called a veggie filter. You can see it in the photo. It's the small horseshoe shaped area above the actual pond. Basically, I ran a water line into the bottom of this area so that water would flow from the pump into the bottom of the veggie filter. (Your system would be less complicated than mine, for sure.) The water flows up through the gravel and then spills over the wall into the pond.

For your system, I can see it being very easy. You would need a large sheet of sturdy, rigid plastic or fiberglass. I immediately thought of that corrugated fiberglass roofing you can get at Lowes or HD. Cut the sheet to fit the inside of your pond, making sure you don't have any sharp edges. The close the fit, the better because you don't want gravel falling in around it too much. Maybe make a paper pattern of the pond interior a few inches up from the bottom.