Hi and welcome to the Chiari forum.
I have to echo Frankie's suggestion as a person having Chiari I do not see how one could possibly participate in a sport such as boxing...it is not safe, but for those with Chiari it can do so many other things too....slow healing, cause a syrinx, and that would be more obstruction to CSF flow if one is not there already....
Those that box can develop issues when they do not have Chiari...so, I am looking from that prospective...
We do have a list of activities to avoid with Chiari and Syringomyelia...it is in the Health Pages -
http://www.medhelp.org/health_pages/list?cid=186
There is much more info in the Health Pages u may find helpful...should u have issues navigating let us know : )