Aloha,
I hear you there. Just wanted to mention it and work from there, so there were no surprises/disappointments for you later on. Depending on the level of isolation you need from aircraft/auto/trains/neighbors/forklifts, etc. you may be able to get by most of the time.That would be a good idea. However, that would start getting very expensive, estimated cost to do that outer wall is a little over $3000, when insulation and drywall are taken into consideration. I would love to do it, but it definitely won't be on the top of my priority list. Plus that really is something I could tack on afterwards, when budget permits it. That was the only disadvantage of using warehouse 2, it has more space, but warehouse 1 was insulated. We all make our sacrifices.
I believe the skp "skin" used was OSB, but the actual materials you can use are MDF or plywood. The denser the material, the better, to help eliminate it from resonating, as well as sagging over time. Aloha 8)I noticed though that the boxes, and base for the speaker look like they are made out of a different material than the MDF fronts that go over them. Is this the case, or should I make the box, and base of the speaker stands out of MDF as well?