I cast off using a stitched cast off, because it is the best I know of to give adequate stretch for such an expansive piece. It is not the fastest, nor the most structurally satisfying, if that makes sense, but I know I can make it stretch far enough to not pull in the end.
The excellent and generous Diana Sullivan has a video demonstration
1 Knit a final row in Stocking Stitch, with the carriage finishing on the side most comfortable for you to start stitching from. I am right handed, and for me, that is the left hand side.
2. Break the yarn, leaving as much as is manageable to stitch with, and thread a suitable blunt needle. I favour the double eye tool. Officially it is part of the ribber set up, but is the single most useful piece of machine knitting equipment. I buy them by the handful.
3. Stitch across the end of the knitting very loosely, starting a new thread whenever one runs out. I needed three.