A Manfrotto 500 series video head would be a great project, and works very well with any kind of photography. I had my tripod and ball head stolen last year and when I bought new, went with the MVH500AH and don’t miss the ball head at all. The only downside is you need an L bracket for portrait orientation with a video head.
Another alternative that might be easier to build is an Acratech Long Lens Head
The Acratech Long Lens Head is your solution for a strong and light tripod head. The indexable clamp will allow you can also shoot with your wide angle lens.
www.acratech.net
For the Arca Swiss plates, cam clamps might be nice and convenient, but your plates must be consistently wide, otherwise they might not hold securely. I use screw clamps on mine and have made several plates for my lenses, but haven’t made a clamp yet since I can get them for less than $20. For the dimensions of the plate dovetails, Really Right Stuff has the dimensions on their website, that is what I use when I made mine.
https://www.reallyrightstuff.com/dovetail-standard
For deciding on how to make things, research patents and product manuals, they usually have drawings good enough to get the design process going.