The RF uses a wave length around 122mm. You want to avoid mesh that uses a multiple of a 1/4 wave length. So mesh where the wires were about 31mm, 61mm, 92mm or 122mm apart would be bad.
So using a mesh with a spacing of 45mm, 75mm or 105mm should be ok.
Also the signal drops off exponentially. So if you could stand the mesh off the AP (say) 100mm that would be good.
Personally - I think I would go for a simple toughened ABS plastic case, like an electrical switchboard case.