On the ones I've made (BC 112, BC 6" micro, my own Plenty), I've rounded the outer edge with about 12mm radius and left the inner edge as is. The micro cab (leatherette covered) has a cheapo round speaker-sized grille, the BC 112 (Tuffcabbed) and Plenty (leatherette) have metal grilles, and I've got a piece of aluminium grille for the 8" cab. https://www.ebay.co.uk/itm/362491694497?var=631653707862