Start a marathon game with the fill start set to 15. You'll want to use LB to put a line piece in your hold, and then use the upcoming blocks as filler. Make sure not to clear them so you can get to the 20th, topmost line of the Matrix. Try getting two blocks on the 20th line ready, you could use a L piece, or a block, anything that fills two blocks, and use two lines to fill it in. If you're lucky you can use other pieces but these are the ones I used. I popped this on my first attempt using this method and blocks. But of course, it's easy to restart if needs be thanks to using the fill start.