Steve...
I've decided to skip Decfest this year! Because I can already see it coming... if I attend, I will think I have to have a pair of at least one of these OB's you're developing. Just what I need.. another set of speakers to add to the 9 pair I already have...!
Portable damping:
I have found these to be quite handy for numerous things... in addition to what I originally bought them for - my other hobby.
https://www.caldwellshooting.com/rests/shooting-rests/lead-sled-weight-bag-stand...you can fill them with lead shot, steel shot, sand. etc. I've even used polymer beads if you want some damping/vibration absorption with less weight. Once filled you can easily move them around and place them in different locations to see if adding mass/damping makes a difference and where it might be most effective.
For those of you contemplating brass products from Eden Sound - don't hesitate. I had him make me up some custom brass footers with interchangeable feet - rounded (half spheres) and tall carpet piercing spikes. He was nothing but professional, courteous and speedy. He fixed me up with a set of footers that are a combination of his Radiused Bearpaws and his Deep Carpet Penetrators - I can change between the radiused bottom and the pointed bottom.
Mapleshade is also another source of brass footers/weights... I bought a pile of their products used off of Audiogon.
I almost always mount drivers from the back of the baffle, especially in my Trio baffles which use a secondary baffle/mounting plate to attach the driver/s to the main baffle. This allows two things:
1) relatively quick changes of various drivers - just make mounting plates for each set of drivers. I currently can run my Audio Nirvana cast frame 15's, 12" Alnico's, 8" Betsy's or a 8" Betsy W.O.W and Dayton AMT combo just by swapping out the drivers which are mounted to their individual mounting plates which are then attached to the back of the main Trio baffle. If any of you have Hawthorne Audio OB's... you can likely do this.
2) you can play around with the "tension" that the driver is mounted to the main baffle with. Kind of what Steve is talking about making rigid clamps to hold the drivers to the baffle. With the Hawthorne Audio Trio mounting method, you can vary the material between the driver, the baffle and the mounting plate, as well as the tension/how tight you attach the driver/mounting plate to the main baffle.
I have typically followed the original designers(Darrel Hawthorne) practice of isolating/damping the driver/mounting plate from the main baffle. I have used various materials between the back of the main baffle and the driver mounting plate and sometimes even between the mounting plate and the driver. I tend to try/experiment with whatever I might have laying around the shop leftover from other projects.
I have used closed cell neoprene foam, cork, Hush Mat
https://hushmat.com/ (a product similar to Geno's Rockwell stuff), Corian-like hard-surface cabinet top material, masonite, concrete foundation gasket material (a heavy duty closed cell foam), particle board, OSB... whatever leftovers from other projects I might have laying around that looks interesting. No point in hauling the stuff to the dump if I can find a hobby use for it.
Just my experience, everyone else's may be different, but attaching drivers directly to baffles has induced very undesirable buzzing/resonances that originated from various places on the baffle. And this was with my Trio baffles which weigh over 100 lbs and have sand filled side rails and are covered with two layers of felt front and back. I suppose with enough experimentation, you could take advantage of this resonance if you can get it tuned to a desirable frequency and at an appropriate level. I've had better luck (and faster) damping/absorbing/moving/reducing/draining away the vibration/resonance that caused the audible buzzing of the baffle structure.
You can get cork from Bangor Cork -
https://www.bangorcork.com/you can get high quality cork from these guys in all kinds of thicknesses, widths and lengths, rolls, etc.
I have purchased sheet and roll cork from these guys in various thicknesses - good product, fast service - no hesitation.
I use a layer of cork on top of all the shelves of my equipment racks.
The manufacturer of the Betsy drivers recommends cork on the back side of an OB baffle. I have seen it mentioned it other OB designs as well.