I would simply add a couple more rows of blocks. The blocks look like they are 10" with all the borders. So you could make the quilt 5x7 blocks and you'd have a 50x70 center. Add the same borders they do in the pattern, and you get 64x84, which is pretty good for a twin size bed.
Or if you want more overhang, 6x8 blocks plus borders would be 74x94".
In either case, you just need to buy enough fabric for the extra blocks, plus some additional for the longer borders.