I was going to do the same. Have my trans guy build one. After looking for the right parts and info, it would have cost me more than buying one from one of the shops that build them all the time. Check out
http://bowtieoverdrives.com/index.shtml. They have a lot of good info. I bought mine from them, level 3. Works good so far. After having it a while I decided to put a switch on the 12v so I could also manually lock/unlock converter when I wanted. I basiclly lock on highway and not around town.