I have a picture in my head of what you are talking about. Just a "thin" traditional style European woodworker's (cabinetmaker's or patternmaker's) workbench without the legs. I say thin, as I usually see those 3-1/2" to 4" thick. Like a Holtzapffel, a Roubo and about a half dozen others that came from different regions...
Funny that your father-in-law probably had some along those lines around somewhere....
Here. Free plan designed by Frank Klausz: http://www.gorillaglue.com/Portals/0...ch%20FINAL.pdf
Note of that plan's "pieces" on the ends of the table. Those add strength to the table laterally across the table.
I got that your holes where 3/4" square and that you were using the f-clamps, but the larger holes- 2"x6"? Festool f-clamps are made exactly for that application (3/4" round holes). I use my welder's f-clamps (a lot beefier).
What works great for doing those square holes is if you cut the length of the square holes as 3/4" deep x 3/4" wide crosscut dados in the glueedge of one of your timbers as you are building and assembling the table. That way one edge of the holes is the gluedge.
Question- Why square holes? Square is traditional... Square dogs in square holes. But not really "required," unless the clamps fit through a square hole easier by twisting them as they go through.
3/4" round holes also work good for those clamps. Many modern designs of those workbenches use round holes, as I think it's because you can just bore a hole and poof. You just then use round dowels as dogs. If you are using take-apart f-clamps to go through that way, you don't need the twist to go through the hole. A holdfast will also work in a round hole (actually better than using in the original square holes), except in your application most holdfasts seem to want to hold better if you have about 2"-4" of thickness in your table.
Edit-- Saw your drawings "after" this post. Hmmm...
Call me tomorrow when you get the time... Besides, I haven't heard your voice in a few weeks!