Like many of you, I frequently use my vehicle for a blind. Typically, I use a bean bag (Skimmer Sack) with the lens mounted on a Skimmer plate, or sometimes I use a Puffin Pad and shoot out of the driver's side window. I usually have 2 lens and body combinations with the one not in use sitting on the passenger seat. Recently, I've lost out on several opportunities to shoot subjects from the passenger side because I didn't have a way to support the lens-camera properly. Window mounts such as those made by Kirk or LL Rue (Groofwin) are not long enough to get enough angle from the drivers side but their design is essentially what I'm looking for albeit with about a 22" plate to mount a ball head, and lens-camera.
RRS, or Kirk don't do any custom work, and the minimum charge for the local machine shops I've contacted are in the thousand dollar range.... much too steep for me!
After a few minutes of sitting in my SUV, I pulled out the tape measure and then spent a couple of hours in the garage with some spare wood. Adding a Manfrotto SuperClamp and a riser block and my Arca-Swiss Z1, I now have a workable solution for shooting from the passenger side, at least with a 400mm f5.6. I have not tried it yet with the 500 but it should work. And, yes, those are Realtree Max4 HD seat covers...