I have the through block on the side deck, but instead of going to the floor then the thwart, I fixed through blocks just above the thwart. Getting the position and angle of these is tricky, but it was possible to avoid the web and the bouyancy foam by placing the blocks toward the rear of the thwart.
It would be as well to check where the bouyancy is in the boat before committing, I guess it might vary.
I also use a continuous sheet.