(Aw sorry MARK, I can't put photos on because I don't have a scanner.)
Basically, I just get the largest plastic soft-drink bottle I can, drink the contents (heh heh), cut the base off, turn it upside down, shove a bit of sponge rubber into the neck to hold in the filter media while letting water run through, half fill it with those "filter noodles" (sintered glass or whatever it is) and put some filter wool or floss on top of them. Suspend it over the tank with wire or chain and (this is the bit that costs a few bucks) buy a small pump such as is used in those little internal filters to pump water up into the top of the bottle. It trickles through the wool and the noodles, aerating as it goes. The only maintenance is replacing the wool when it gets a bit dirty and occasionally pulling out the pump and flushing it.