Hi all. First post here. Thought I'd try to contribute just a miniscule amount back to the community, by way of sharing what I've discovered about suitable plywood in Canada.
Rona caries the recommended Arauco ACX stuff. At least they do in Alberta. 1/2" runs $35 a 4x8' sheet. I had to cherry pick through about 8 sheets to find 3 I liked, so that's about the quality percentages one can expect from it I'd guess. It's not what I'd call completely void free, but is very nearly so and I think will work fine.
Here's a couple pics:
Cuts stacked and ready to go:
Also, Windsor Plywood, for those in western Canada (or Thunderbay, ON) can bring in 4x8' sheets of Baltic Birch if you ask them. $60/sheet. Overkill for a concealed THTLP subwoofer, I thought, but is (as many have already stated) very, very good stuff.
Cheers!