There is NOT a Slifer playmat. At least that's what I think. I got the Ra playmat flip summoning it 10 or more times per duel so I tried the same with Slifer. I flip summoned Slifer 117 times and got nothing. So maybe konami changed the requirements or something, making it count only if you Tribute Summon it, or maybe there is no playmat at all. That's all I got to say.
Same here... I have built the deck with Gonogo but it seems to not work with Slifer.
Has anyone already unlocked that mat? At least to know if it exists or not...