Thanks Judy and Beth for your many suggestions. VERY much appreciated! Judy, I, too, have always done a lot of my best thinking while driving.
Beth, Joe has definitely tried the "bathtub option", although he is not a major fan of actual baths.
Joe, like most dogs, is very gregarious. Aside from the breathing/heat problem (a dog's natural body temperature is 102 degrees, so an unvented, insulated box might soon warm to uncomfortable levels. Which is why - shhhh! - they are so nice to cuddle on a cold night! ) then there is the sensory deprivation problem.
To confine Joe into a smallish, or even a largeish box, without a way to see out might be more upsetting for him than the noise, no matter the treats and comfy bedding offered.
Think solitary confinement for a gregarious individual.
Comfy bedding is also out for Joe, any kind of bedding in his crate (from towels to blankets) he immediately tears to shreds (I'm not sure why - all of the dogs I have had over the years loved a blanket to snuggle in), with the very real risk of his injesting bits of it.
To put ventilation holes in such a box would kind of defeat the purpose of keeping out the noise, I think.
Finally, my carpenter skills are not up to the task, alas. My major goal is to prevent him from injuring himself as he does something stupid, and to prevent damage to my belongings (think flat screen tv smashing to the floor as Joe tries to climb behind it). Confining him to his regular crate accomplishes both of these.
Truly, confining him to his crate (it is an extra-large one that originally belonged to a 100 pound GSD that I had long ago - Joe is about 45 pounds) does seem to work better than anything. I do need to remove his food dish and water dish while the noise is going on (if the water dish is left he steps in it and spills it everywhere anyway during a "noise event"). I turn off the light in the crate room (but leave the room's door open) for however long it takes for the noise to stop.
I have tried cuddling, which works just until the next major boom, then he squirms away. I have heard of the Thunder Shirt (thank goodness that, in general, thunder storm is a fairly rare thing here, and even when they do occur they are nothing like the severe midwest thunder storms I experienced when living in Ohio. I'm thankful for small favors!) but, for Joe, I don't think the shirt would work too well.
In any case, the booms have mostly stopped now. I hear random booms here all year round, but the worst are 4th July and New Year's Eve.
A little less than six months to go until the next "event". I can't wait.
Thanks again everyone for a fun thread!
Anne