Hello all. I'd like some input from the bog gardeners. I'm going to create one from a hard plastic pond tub. It is several feet deep (at least two to three, if I had to estimate). The tub will be above ground. What is best to use as a base for the bog? I could fill the whole thing with the peat mix, but would like to find something cheaper (and not requiring mixing), perhaps gravel or sand. Anyone have experience or ideas? Much appreciated.