Advice needed on FRP standoff box (bracket)

[fede]

Hi guys, I need some good advices.
I'm trying to figure out how to make a good strong standoff box directly from the mould for my 26 leisure RIB.
The question is : with a shape like that (with almost perpendicular surfaces) how should I design the structures ??
I usually make transoms like this : FRP,Wood core,FRP.
Now I would have to cut a hole in the wood core because obviously the insie of the box should be accessible from the inside for jackplate mounting reasons.
The Box should be 10 inches (25 cm) long 16 inches(40 cm) high and 16 inches wide.
How should I reinforce corners and box inside as well ??
Rhino pic attached.
Thanks in advance.