I will be building two 15" Dayton RSS390HF 22" 4.3 ft^3sealed cubes with a iNuke NU1000DSP. Here is a link to the box design: Model. Designing a sub is pretty straight forward as long as you have tools or someone who can help you cut the wood. If you don't want to use a pro amp you can always go with a plate amp from Oaudio or Parts Express.
Partsexpress offers kits that include the box. You'd still have to assemble it yourself but it saves you the time and effort of figuring out your measurements and cutting all the material etc.
If you're building a sealed box then it's easy enough to do yourself. You can even have a local shop cut a sheet of ply or mdf to size for you. Grab a tube of PL Premium and a drill with screws and you're ready to assemble.
If you're going with a build from scratch, try to use the MDF that was already mentioned since it's denser and won't resonate as much. It's also easier to work with in terms of screwing pieces together, durability, etc.
If you find you're not getting what you want out of the sub, adding fiberglass or polyester stuffing to the box can help sharpen and improve sound and bass and is pretty affordable. If you go the fiberfill route, the rule of thumb is one pound per cubic foot of airspace but of course, what sounds best to you is what you should go with. Ears don't use calculators to decide what they like