The best way to do it is to run two bars across the back of the van, about 8 inches apart. Mount the reel to that. Then weld a frame to hold the drill together. Its quite simple, I will post a pic of mine tomorrow. It holds the drill in place and prevents wrist twisting if the hose snags. Of course all the shock is in the drill then so metal gears is a must. I ran mine like that for a year and the drill is still going strong, just the batteries are a bit dodgy now.
i would be interested to see your set up. i currently use a drill but just hold it in my have, but have been looking for a way to fix it in place.
also i run my drill from a 12v car battery, that way you do not have to worry about it going flat, its been that way for 3 years