the runes i had the same idea to do it also on the glyph to mix it with attack speed and magic resistance scalling per level and for armor i wanted to do armor plus health scalling , but you have hose to use attack damage, one thing is your build is 5 items 1 item is missing xD

As a Kog'maw main, i'd ask why are you picking Armor pen in masteries if Kog'maw does more magic damage than physical damage?. Physical: the barely 100 damage you do on auto, Bork; Magic: Q, W Max.Health damage, E, R, Wit's end, Rageblade, Ardent (if one), Zeke's (if one). Also you better go for Stormraider's surge, since Kog'maw isn't mobile but can burst easily, so it'll give him a mobility boost to escape/stay safe.