Well, I built one a few years back. It worked. I ended up buying something that has had hours upon hours of testing, fine tuning and worked like it was supposed to.
I would say your best bet is to copy something that is already out there and will fit in your space, that way you know it works well and you won't end up wasting money. Plastic tubing of any size isn't cheap and fittings, bonding agents, parts....
http://www.tapplastics.com/ is a good place to get supplies.