I don't know if they were canvas or cotton canvas, specifically. As for the squares, the large ones were 5 1/2 inches wide by 7 1/2 inches tall, and the small blocks were 5 1/2 inches wide by 3 3/4 inches tall, roughly. While ANTR's jackets were more visuall accurate, Cameron had the "block split" correct. In pictures of Titanic's lifebelts, each whole block appeared to have a groove in it or perhaps even 2 blocks in each pocket. I would imagine this was to give the lifebelt a little more flexibility.
I didn't make mine out of cork and canvas - more for visualization purposes and curiosity, I just used foam blocks and sheet. I don't even know where one could get cork block.