Advice on hollowing out a slit drum

I am a beginner at carving and I do not have any one around to show me what I'm doing. I am carving a toere drum. I have the slit carved 18' long and 2' wide over half way through the drum already (about 5' deep).
Now the problem.
How do you carve the interior hollow using only the slit for access?
Any advice would be great.
I have only started to carve, did a wood fish hook from koa and now starting both the toere and tradition pahu drum.
Big ambition.

Pete, welcome to TC. You might want to post your question in the carving forum - people in this forum are not allowed to own sharp tools (conditions of probation) :)

Thanks, I'll re-post over there.