Hi Sylvester
Nice looking car with a few nods to the ariel atom and donkervoot going on there!
With regards to one piece you can do it,and there is some technology available from FAR Uk / axon automotive that use a foam core with a braided carbon warp around it.
But if you want to try it yourself you will need to have rohacell or similar core machined to the shape you want then wrap it in your composite and infuse, i would suggest you make a tool or some caul plates to give you a nice surface finish.
To bond both sides you will need to have a suitable joggle designed into each part or a return flange to give you a decent amount of bond area for joining the two parts. There are plenty of structural bod glues from3M and other companies. 3M 9394 or 9323 are very good for such applications.