If you're mainly looking for acoustic purposes and only want rigidity as a secondary, I'd look for the Handi-foam 2lb kits. They go in way easier since the kits are pressurized, and the nozzle is very small so it can go into small holes.

For most members, you just use whatever available holes are there. With the Handi-foam kits, you can even shoot it through the small holes for plastic clips. With the pour type kit that I used, you have to have a funnel. I had to open up some holes to 5/8" to get my funnel in there.

As far as fire, I think it's flame retardant. It's really supposed to be for boats, so whatever the Coast Guard requirements are.

It's also fully closed-cell with over 99.9% moisture rejection. I've heard a lot of people talk about possibly promoting rust. That's a bunch of crap. I sprayed some spare Handi-foam in a pile in the yard once. It sat out there for 3 years and hadn't broken down or gained weight from dampness.