I'm thinking that the way to go for Mega Snorlax would be to give it a brand new ability - I'm thinking an ability called "Fat" which is basically Fur Coat, but trading the ability to 50% damage from all physical moves for a the ability to take 75% damage taken from both physical and special attacks. Like Fur Coat, it is negated by Mold Breaker, Turboblaze and Teravolt. Like this:
Snorlax --> Mega Snorlax
Typing: Normal --> Normal
Ability: Immunity / Thick Fat / Gluttony (Hidden) --> Fat
Stats: 160 HP / 110 Atk / 65 Def / 65 SpA / 110 SpD / 30 Spe --> 160 HP / 180 Atk / 85 Def / 65 SpA / 120 SpD / 30 Spe
I get the idea behind Arena Trap (or a clone that fits the theme of Snorlax), but I think this makes much more sense for a Snorlax than any trapping ability imo. I'm drawn to either this or that attacking in sleep ability suggested by
Mega Mantine as the other sleep-based ability (the one which is a rest every time you switch out) is literally broken. A full heal when you switch out is just too OP, and it doesn't matter if it makes it "vulnerable" because it is still a Rest on every switch, making it basically an OP Regenerator.
Also, Slack Off on Snorlax makes sense at a glance, but then you realize that Snorlax does nothing but sleep and eat until you use a Pokéflute and capture it, so it isn't technically 'slacking off'.