I haven't played this matchup much in a long time but I'd think you'd be better off trying to utilt or dash attack him first, perfectly executed glide attack is -3 on block so it's not actually punishable for snake unless you powershield it or the MK does it wrong. I know as marth I just fsmash or bair his glides, shielding gives him some other gay mixups too(land behind and grab etc)
not sure if either of those would work but otherwise I'd just avoid the situation, reverse pull a nade when he's gliding at you and GTFO
aside from that most MK's habitually cover blocked glide attacks with something you can **** them for if you hard read it, what is he doing after, dsmash, another SL, nado, roll, spot dodge?