I forgot to add with my pictures that I use lava rock on the bottom then a grate, which is just an old clothes basket sides cut and twist tied together then my chair padding basically from walmart cut to size of my tub which is 2" thick ( two of them on top of each other) which fits just under my 2" pvc pipe outlet that I siliconed into the side of the tub.
My pond is only 270 gallons but this works great for it as I clean out my filtered materials once a month ( I just use my garden hose and I live in the city it doesn't seem to bother my fish at all). My DIY filter flows into my homemade wier (which is just a hard plastic bin). Here's a pic of how I hid the whole thing with plants in my "fake" bog ) and my grate.