can we get more info on the pond is it a rubber maid or in the ground ? what do you plan to make the filter from ?
give us this and we can give you better advice.
Easiest would be an in-pond filter like you drew, a box with the pump in the center, surrounded by gravel, surrounded by course open cell foam. Very simple and will handle a 300g pond nicely (unless it is highly over-stocked with 24" koi and turtles in which case we're talking several 55g drums with a lot of bio-material and a 3400gph pump ). But let's take it one step at a time.
the pond is a inground pond. made out of cement. it was there when i bought it. it has a big waterfall wall built in too. i will be stocking with common goldfish, pleco, comet goldfish, bullhead catfish. i was planning on making the filter out of a decent size rubbermaid. and i was planning on using 1 pump. i was wishing for a filter that would be out of the pond but if 1 pump isnt sufficient than i would settle for a inpond filter.