Do you need to accurately hold the sheet metal pieces together during rotation? If so then you'll probably need at least one bearing. Does the center bolt need preload only to keep the nut from falling off? If so then a shoulder bolt and nut would probably be enough.
I think you want a notch filter. Piezos are generally good at high frequencies (high output) and not so good at low frequencies (low output). I don't think you want to build a notch filter with them.
Are you looking to filter a "drive signal" for a test rig? When I did this kind of stuff...
A lot of work but... build half the circuit on the PCB and the other half in the breadboard. Add/subtract the components to each until you get what you want. That will at least isolate what section of the circuit isn't working as expected on the PCB.
Just FYI, McMaster sells angle brackets. They have large filleted radii (no welds). They are also cast iron (damps well)
https://www.mcmaster.com/products/angle-plates/angle-plates-2~/
Bob
Only kind of related - you'd generally want to mount the specimen the same as real life. Looks like yours may be glued on? That will change the frequency response.
With turning you can usually use the same tool for OD turning and facing. Also, if you have a tool changer on your lathe you can bring in different tools (like a grooving tool for narrow grooves). You don't have to true or dress a turning tool (though you might have to replace it when worn)...
Backlash in the connection would be important I'd think. The only one of the three I've used are the ball locking pins - I wouldn't count on those if you need a pre-loaded joint.
How are you distinguishing "moments" from "Torque"? In the first figure the shackle will rotate (as SWComposites says) until there is zero torque on the bolt axis. There will be a moment on the red base though.
I addition to the above your options would include torsionally soft or stiff. I haven't built a generator/engine machine so I can't tell you how they are normally done.