Do you want a box built and shipped to you put together or do you just want a blueprint to follow so you can build your own?
Mark from car audio fabrication will design a custom enclosure for $55-$85. I did this for my 12" zv5 and I just ordered another for a 4th order truck bed blow through. They are easy to follow, sound great, and don't break the bank.
A middle ground option is GP audio. They sell glue it and screw it kits. Basically pre cut peices shipped to your door ready to assemble. It's more expensive but if you have never built an enclosure before and don't have any of the tools required to do so, once you buy all the tools and the wood, this might be a cheaper option than going with Mark.
Lastly there are companies that will design, build, and ship a box directly to you. The enclosures are pretty expensive. I've seen basic 12" designs go for $250. Add a little plexi glass and a few other custom touches and your talking $500 easy to get a single box built.
As a side note, all of these options are cheaper than going to a local audio shop. Me and my brother paid $1400 when we had our blow through put in orginally. Granted this included cutting a hole in the truck, designing and building a bad ass custom enclosure, build a beauty panel, and install/tune the amp.