<3 the Snake Vs MK matchup. It's so dynamic and crazy. Unless air planking or major camping is involved, but I rarely come across it.
Let's see...OOS options for perfect shielding MK's Up B...Well, it depends how smart the MK is. Pivot grabbing, as said, can work from behind the MK if the MK is dumb enough to always dive right down after the UP B into a Glide Attack. Moving a little forward and utilting can also work, AND can KO at high percents. If the MK is really stupid, Bair/Uair/Nair work. And of course there is regular shield grabbing it after taking a few steps forward so the MK can't space the Glide Attack away from you to avoid the grab.
The smarter MKs may see this coming and glide right past you without moving down. If you can't snag them with a hit in time, you can just dash attack the MK from behind. I do it all the time, it works fine. I guess you could try to go for a utilt or Bair/Uair etc but that's only if you know they'll do it before hand for the most part. But out of all the MKs I play around here, only ones like Ksizzle, Diem, and Fatal generally do that. In short, if perfect shielded the Up B, best option IMO is: Take a few steps forward very quickly and shield grab the Glide Attack. Dthrow and lead into a tech chase 95% of the time unless a nade is nearby or a Bthrow will KO. If they pass by you, hit with dash attack if not fast enough.
For Dsmash, ftilt/utilt/grab are your only reasonable/best options. And I guess dtilt if you think it'll KO or dash attack. If perfect shielding it from back OR front, it's a free ftilt from any range and up close enough gets you a shield grab to Dthrow tech chase. I GUESS Dair could work too, but that's stupidly unnecessary. If not perfect shield, you have to be careful of your distance to punish it. If not close at all, an ftilt may not even reach in time before MK can shield, much less a grab. If you're too far without perfect shielding, best to back off or you could just eat a Dsmash.
I was bored xd. Snake has a lot of options for dealing with MKs moveset though, just a matter of finding out what works best.