Did you set it to MATERIAL_CULLMODE_CCW? If you set it to MATERIAL_CULLMODE_CW, it doesn't do anything at all because that's what its set to by default.
ent = ClientsideModel("your model here", RENDERGROUP_OPAQUE)
ent:SetModelScale(Vector(-1,1,1)) //to flip on the x-axis (I think that's what you want, if it doesn't work try the y-axis.
EDIT: Or is it the other way around? I am confuzzled today.