If you want to design your own (since your Bogen tripod should be perfectly adequate for 4x5")--does the camera use a cylindrical rail? If it does, I like the way the Sinar and Linhof pan-tilt heads work for their monorail cameras that have cylindrical rails. The head is simple and just tilts and pans, and leveling is done by loosening the rail clamp, adjusting and tightening. This keeps the head light and compact and low profile.
The downside of this design is that you have to turn the camera 180-degrees on the head to point it downward. If you want to have the ability to point it down without turning the camera, you might make a kind of Z-shaped head with two plates hinged on opposite sides--one for tilting up and one for tilting down.
If you are using a square rail, you might look at a thing called the "Levelhead" for inspiration--also a two-plate design, but hinged at 90-degrees so you can level the camera fore-and-aft on one axis and left-right on the other. It's a simple thing, and there should be pictures on the net, but Louis Shu at Photo Gizzmo carries it, if you want to see one in person. Instead of using it just for leveling, you could make it with a wide adjustment range as a full-function head.
