// rail base
translate([-rail_ex/2-wall, -lens_d/2-rail_depth-2*wall, 0])
cube([rail_ex+2*wall, lens_d/2+rail_depth+2*wall, body_l]);
// rail base
translate([-rail_ex/2-wall, -lens_d/2-rail_depth-2*wall, 0])
cube([rail_ex+2*wall, lens_d/2+rail_depth+2*wall, body_l]);