I did not line my box, as I didn’t find any sharp edges on my plastic canvas, but those who are using this with small children may wish to do so, or to substitute a cardboard box instead of the plastic canvas. I bought my plastic canvas from Michaels for $1.49 per sheet for 10.5 in x 13.5 in.
The plastic canvas, even the extra sturdy kind, tends to bow out a bit when the box is full. Lining the box and further reinforcing the corners would help this. I’ve also considered making one in 9 x 9 inches to match the other boxes in the series. The pattern would be pretty easy to adapt to those measurements.
I used 250 yards of red and 200 yards of off white. Other knitters may need more. One tester needed 380 yards red (3 1/2 balls) and 240 yards of off white (2 1/4 balls). Cost of materials about $20.