I just made one for my 4x2x2.
They are pretty basic really, the main thing you need to consider is how to get the water out of your tank into the sump via gravity. Once that is sorted they all work basically the same.
My tank has a corner overflow with a bulk head at the bottom which makes it easy. If its a new tank (ie. not yet filled with water or fish) I'd get it drilled and fitted with an overflow whilst you can, otherwise you will have to set-up a hang-on overflow which is bulky, hard to hide and uses syphon action which is more problematic than an internal overflow design.
The actual sump filter I made consists of a 30" aquarium from the LFS, I got them to cut me a glass baffle at the same time I purchased it, along with a couple of extra glass strips to use as supports for the drip tray and and a sheet of eggcrate mesh which the bio balls sit on.
I divided the tank in 2 with the baffle (2/3 for the bio balls, 1/3 for the return pump and general sump area for heater, etc.
I will try and get some photos of my set-up soon as its a little hard to explain without pics!
Here is a link to a wet/dry DIY, mine is exactly the same except mine only has one bio-ball chamber where his has two. I made the drip trays out of 4mm acrylic sheet.
http://www.piranha-fury.com/information/default.php?id=wet-dry
My wet/dry sits under my tank but the tank stand does not have doors so its visable. To prevent it looking too ugly I used automotive window tint to darken the tank and make it little harder to see the internals. You can now only see the internals if you look carefully, otherwise it just looks like a large black cube!
