Not sure what you mean by topology? If you mean Tee or Pi type you may be surprised that most are neither, relying instead on a 2D slab of resistor material (which ohms out properly as if it were Tee or Pi). The big trick is broadbanding techniques especially when trying to ground the shunt. I would need to find an old paper to explain this type of attenuator. Madengr should know.

YIG is a very expensive single crystal. Yes the sphere would resonate but to "see" it at 6 feet would probably require gain (dc power, cost, etc.). I guess cost in quantity >~$40.

Buy and plate standard screws. However, when you make the cover (or whatever) with the threaded holes use an oversized tap to ensure a fit after cover plating (i.e. adjust the taps pitch diameter). This is how we did it in combline filter world. You can get almost any p.d. you want. I stripped a 2-56 hole and had it tapped up to 3-56 to save an expensive device. A good designer can figure out the appropriate p.d. depending on screw size and plating thickness. In contrast, screw manufacturers probably wouldn't make nonstandard screws unless you were willing to buy maybe 100,000 of them.