Steve-
A fellow built a massive hidden shower filter that could be adapted in smaller size for your pond's spot. Try Koivet and search CarlosKoi for the thread on his pond build. No idea if it is still there, haven't been there for years, but it should be there. Stacked milk crates (cheap enough) filled with matting and lavarock (cheap enough) hidden by a slate covered wall ( a little bit of industry and cost involved there, eh?

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At my house I have a 4' x 2' waterfall built of 4" solid blocks and waterproofed. I run from my skimmer to a 1200gph pump and up over this 5 cu. ft. of media. It works. It was cheap and I'm cheap. I think it looks good. Improved openness of structure for aeration of the media, more height and more flow are all changes that would improve its performance.
Mickey the windowman