[AlexH]

Do You think there would be any value to having sand filled panels in an OB sub?  Overkill or not?

[Early B.]

Yes, there's benefit in an inert cabinet, regardless of an OB or sealed sub. For OB, where would you put the sand? Maybe affix marble tiles to the side panels. That will stiffen it up quite nicely.

[AlexH]

I was thinking both sides, top and bottom. The panels would be 2 1/4" thick with the middle 3/4"  of each panel being filled with sand. So 3/4" mdf / sand / 3/4 mdf. Panels would be made before assembling.

[ThePriest]

Agree with Early B. Inert cabinets are good.
I glued 3 mm steel sheets onto the sides and top of my GR sub towers.
I used a non-hardening marine glue, with 2 mm spacers to attain a uniform glue layer.
The end result weighs around 70 kgs each, and there is barely any vibration when playing Kraftwerk...

The steel panels had been lying outside, covered in sand and acidic acid for some time (maybe a bit too long, to be honest), to give a 'natural' rusty look.

/ThePriest