I needed to reinforce the current shelf, I designed a pair of struts. First, I attached the "Supports" part with wood screws (3 mm diameter), then I put "Fillers" inside (it is not necessary to glue, but you can) and finally I finished the hole in the front part with a "Plug". Most of the holes are vertical, but two are slanted due to poorer access to the hole. Possibly, it will be necessary to drill the hole for plug first with a 7 mm diameter drill bit due to the subsidence of the layers under the supports.