I've been modeling GT wing for a car to 3D print and I want to model the inner structure in order to print in 4 separate pieces(since it is too large to print in one go). And I've been trying to make it so that the wing has a thin wall but hollow inside for material and weight saving that looks something like the picture below but with the GT wing if that makes sense. I've tried converting it to a solid model and made it into a shell model with FreeCAD software and using a modifier like Solidify Modifier to make it a shell-like model for the past few months but none of them worked unfortunately. Solidify modifier seemed to be a somewhat promising method but then a complex model like this makes the mesh intersect with itself when the modifier is applied. Is there any way of getting away with this self-intersecting problem or a completely different solution to make it a shell-like model?
You were on the right path with the Solidify Modifier. Just set the Offset to $1$ instead of the default $-1$. Offset of $-1$ extrudes inwards but your geometry is too dense already hence it starts intersecting so it's best to extrude outwards with offset positive 1.
But of course this will change the outer dimensions and modify the outer roundness. Your problem is those 2
Edge Loops at the top and 2 at the bottom, so I would dissolve those 4 Edge Loops with X > Dissolve Edges.
And add 2 supporting Loop Cuts.
Now you can add the Mirror Modifier and Subdivision Modifier back again.