This is a really, really a BAD idea!
When you over pot, as you will be doing here, there is far too much media for the roots to absorb the moisture from, and the media will just stay wet forever. Do I here 'ROTTEN ROOTS' here? The plant may also spend so much energy trying to send out roots through the media, that it will not bloom for several years.
If you are desirous of a big decorative pot, just up-pot your plant into a plastic pot allowing about 2" space around the edges, as previously suggested, and then place the new pot in the decorative pot. You may have to elevate it with some course rocks or even pots. I would not submerge it into media of any sort as it will impede the drainage from the plastic pot.
I never recommend using more than a 3 gallon pot for Cyms unless you plan to have a block party and BBQ when it comes time to repot or divide you plant!
Cym Ladye