I assume you are transplanting a runner...

Rhizomes usually stay near the surface when growing in unconfined spaces, generally staying within the top 12 to 14 inches, from my experience. In a container, they will eventually grow deeper. If it were mine, I would make it at least 2 feet deep if you make it according to the dimensions you gave above.

If you plant tall or thick culms, the new culms that come up will 'adjust' to a size appropriate for the container. A larger container will grow larger culms, and smaller containers will usually have smaller culms. It is more important to get a good rootball or good bunch of roots/rhizomes than it is to get large culms.

No matter how small you make the drain holes, the rhizomes(if they grow that deep) will grow through them no matter what size they are and fabric usually will not stop rhizomes either. I just cut them off as I spot them growing out of any holes.

I personally like to put the drain holes on the sides, as far down as possible, so that it is easier to spot an escaping rhizome.

Let us know what you decide to do, or maybe post a picture.

Good Luck

Kt