Jan, thanks for your design and your suggestions along the way. Here is my completed, sans binoculars, project. My binoc's came with a window mount at no additional charge. I liked the fact that it has an arm so I modified your mount area to accept my window mount. Basically a rabbit cut with 3/4" plywood so that I can attach it either to the top or the front as needed. I find that 7.5 lbs of weight will balance the tripod. Here, without binoculars, 5 lbs work with the mount & tripod.
The tripod is a heavy duty video camera tripod, shown here in its lowest position. The feet have lockable wheels which are very useful in a parking lot. The head extends a good 12" above the base. The tripod was given to me by a friend of over 20 years, Stanley Pechenick who got it from a friend of his, a former teacher of his who recently passed on, Charles Grunfeld. I couldn't think of a better way to immortalize these long term relationships other than to extend the useful life of the tripod with your design for the trapezium.
This photo was taken at the home of our mirror making workshop mentor, which our club, The Astronomical Society of Greater Hartford. I brought it in for 'show and tell' for the new mirror makers to admire. My new project is a 16" truss tube based Dob, so I've got a lot of upper body workouts coming! Thanks for putting your designs out there. I plan to start on the Poncet next month! John A. Sillasen