I understand your spelling of it. I forgot I'd have to add two ㅅㅅ together, but when I type it, they don't conform to a 3 build like your's does. When I tried it, it went more like 캇시. I guess the question is whether or not to add the place holder or keep it as 시 like in "Si Won." (Sorry, that's the only name I can think of. My mind went straight to Kpop... how sad.)
So right now, my ideas are:
캇시 or 캇싱
Then again, I'm still learning, so grammar and placements are still confusing.
--Edit--
It may be spelled 카시. I happened to come across "Cassiopeia" and it's spelled 카시 for Cassi. We may have a breakthrough. Now, can I get used to being called "Kah-shee"?
*Off Topic*
By the way, if anyone is wanting to learn the letters/writing, I think this site is a great tool also. I believe it reinforces what we can learn here.
http://langintro.com/kintro/toc.htm
I'm trying to get my writing nice and clean, but it's difficult for me to understand how to place them on a notepad (I write big letters in English) but if I write slowly, this site helps me become cleaner.