Why molding ? it seems that grating is awfully thin.
what material would you pour in the mold that can fill these tiny canals ?
I would suggest finding a grating about that size, cut
the grates to size and solder them on 200 pieces of sawed-off
square tube in an oven, like they do with electronics nowadays.
or maybe simply soldering with a torch might do the trick as well.
that grating seems available, a quick google gave me this one as an example : http://www.modulor.de/shop/oxid.php/sid/37501946bcf786279ecfbaccc70e4804/cl/details/cnid/TCS/anid/TPMS/tpl/-/lang/1/listtype/list
otherwise, maybe etching it out of a plate of brass and then folding it into a box shape might be feasible
if you know someone who has an etching bath