There's a YouTube video of a Scottish man making a homemade filter for his duck pond, using pond grasses and gravel and I borrowed his idea for my pond and I'm really happy with how clear the water is. I used a large concrete planter rather than his barrel, but similar filter material.
12" tall is very doable. Just hill up one side, not the drain side, with a gradually sloping dirt hill. Lay your dirt over straw bales just to save money on dirt fill. I've seen fabulous raised ponds made like this, with the makers eventually putting nice floral material all along the edges. If...
How high are the sides? And how do you drain it? It might be possible to build up the one side, where the drain isn't, maybe using straw bales, rocks or loads of dirt to get it built up to the rim.