Trouble thickening a surface for FlowSim

Hi,

First off let me apologise if this is a repost, I searched for a solution before asking but couldn't find one.

I am trying to carry out an external CFD analysis on the bodywork of our FSAE car for a university project, but cannot seem to find a way of thickening the bodywork succesfully. It has been created by another member of the team using surfaces, and is constructed in 4 seperate 'panels'. I can thicken each panel individually (but have to hide its adjoining panels to do so) but because it is not knitted together as one surface the panels do not meet properly and the simulation fails. When I knit the panels together to form one surface and then try to thicken, it says 'Rebuild Errors: Unable to thicken the surface'.

There are some open loops/edges in the design, and I can't figure out how to close them. I have followed tutorials on here and other places (Select open loop>Fill Surface) but still no joy. I'm not sure if this has anything to do with the surface failing to thicken, but thought I'd mention it.

Any help would be much appreciated!

SolidworksFlow Simulation