Rolf,
Some rule of thumb stuff.
A stainless steel mast base need not be more than 4mm thick.

Stainless is 7 times heavier than carbon per volume..
So a mast base from carbon would at the same weight would be 28mm thick. I think 1/3rd of that would be more than enough.
I'd use uni carbon because it is cheap and plain weave kevlar because it is durable and carbon does no like being drilled.
So the kevlar inclusion would be hand to counteract carbons lack of tollerance to being drilled..

I know that the voices in my head aint real,
but they have some pretty good ideas.
There is no such thing as a quick fix and I've never had free lunch!