My 2p worth.
I tried on more than one occasion to make my own bio filter, please allow me to share.
The best box was the black water storage tank, with optional lid, but alas you can't easily buy them any more, so people go for "storage boxes" but often these are not UV stabalised for outside use (They have no need to be) and they split and or crack very easily and do not weather well.
Then you have to buy some pipework even if you are just making a simple spray bar, the pipework is not cheap, and you also need a hole cutter for the pipes (more expense) You can always choose to drill a series of small holes, but get one wrong.......
Once you have the pipework in (not much really) you then have to buy bio media and filter sponges, I was able to take mine from another filter, but if you have none, it is something else to add to the cost.
You may also wish to consider building a larger biofilter with dividers, I have seen this done on the www. It looked the bees knees, but as soon as water went in the walls bowed and de-stabilised all the dividers, the only solution was to add a couple of luggage straps around the outside of the filter, it ruined the whole look, and luggage straps are not known for their durability.
I like what Ed has made
, but in this case, (small pond) it will be cheaper to buy a bio-filter than to make one, I have seen bio filters for less than £30
:arrow: TIP:
If you do want to build your own filter, do use "tank connectors" for inlet and outlet pipes, but remember they do have lock nuts, so they can not be mounted near an edge as the lock nuts need about 2cm clearance.