I made a metal one for mine that takes an A360, which is the commodore filter, but before that I ran a Nissan Pulsar box I got from the wreckers, $40 and with a few minor mods to fit it was great, left the gu round box for dead. As smeagol has done, nothing wrong with a plastic unit as long as it seals.
There are a couple of threads in here somewhere, depends how much room you have also with battery's ect as to what size you can run.
cheers.