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 halve damage from all physical moves for a the ability to reduce damage taken from both physical and special attacks by 50%. Like Fur Coat, it is negated by Mold Breaker, Turboblaze and Teravolt.
It would probably have even lower speed than before and buffed up attack, defense and special defense stats. 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 / 160 Atk / 105 Def / 65 SpA / 140 SpD / 10 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'.