If you don't have one, you should order yours asap. They are made to order in Japan by Zen Sankei and are a once-off cost. Zekken pretty much last forever and you can easily transfer them from one set of bogu to another. The only exception is if you need a smaller size for children's bogu. In that case I would order a medium sized one.
Otherwise for adults, order the large size.
Here is the link: